feat(cycle): choose cervix as the symptothermal secondary symptom

Let a user pick the Sensiplan double-check's second indicator — cervical
mucus (default) or cervix observation — mirroring the published NFP
standard where the temperature primary sign pairs with one chosen
secondary sign.

- `CycleProfile.secondarySymptom` (MUCUS default | CERVIX) drives which
  second sign the engine confirms against. Casual users keep temperature
  + mucus untouched; the choice lives behind the advanced cycle settings.
- `CycleDayLog` gains the three standard cervix signs — position
  (LOW/HIGH), firmness (FIRM/SOFT), opening/os (CLOSED/OPEN) — as nullable
  columns (migration 0157), wired through Zod, the day-log upsert, the
  DTO, the sync feed, backup/restore and OpenAPI. Additive and
  back-compatible: existing profiles backfill MUCUS, existing day-logs
  keep NULL cervix signs, iOS inherits the fields.
- The engine's `confirmSymptothermal` takes the secondary-symptom choice;
  on CERVIX it confirms against `detectCervixPeak` (last fertile-cervix
  day followed by three closed days — the cervix-closure rule) instead of
  the mucus peak. The double-check still requires BOTH the temperature
  shift AND the secondary peak: a single cervix sign never confirms, and
  the still-learning gate and conservative posture are unchanged.

Engine derived clean-room from the Arbeitsgruppe-NFP / Raith-Paula &
Frank-Herrmann standard.

docs/api/openapi.yaml

@@ -134,6 +134,21 @@ paths:
                     - CREAMY
                     - WATERY
                     - EGG_WHITE
+                cervixPosition:
+                  type: string
+                  enum:
+                    - LOW
+                    - HIGH
+                cervixFirmness:
+                  type: string
+                  enum:
+                    - FIRM
+                    - SOFT
+                cervixOpening:
+                  type: string
+                  enum:
+                    - CLOSED
+                    - OPEN
                 sexualActivity:
                   type: boolean
                 protectedSex:
@@ -275,6 +290,27 @@ paths:
                         - WATERY
                         - EGG_WHITE
                     - type: "null"
+                cervixPosition:
+                  anyOf:
+                    - type: string
+                      enum:
+                        - LOW
+                        - HIGH
+                    - type: "null"
+                cervixFirmness:
+                  anyOf:
+                    - type: string
+                      enum:
+                        - FIRM
+                        - SOFT
+                    - type: "null"
+                cervixOpening:
+                  anyOf:
+                    - type: string
+                      enum:
+                        - CLOSED
+                        - OPEN
+                    - type: "null"
                 sexualActivity:
                   type: boolean
                 protectedSex:
@@ -433,6 +469,21 @@ paths:
                           - CREAMY
                           - WATERY
                           - EGG_WHITE
+                      cervixPosition:
+                        type: string
+                        enum:
+                          - LOW
+                          - HIGH
+                      cervixFirmness:
+                        type: string
+                        enum:
+                          - FIRM
+                          - SOFT
+                      cervixOpening:
+                        type: string
+                        enum:
+                          - CLOSED
+                          - OPEN
                       sexualActivity:
                         type: boolean
                       protectedSex:
@@ -730,6 +781,11 @@ paths:
                     - TRYING_TO_CONCEIVE
                     - PERIMENOPAUSE
                     - OFF
+                secondarySymptom:
+                  type: string
+                  enum:
+                    - MUCUS
+                    - CERVIX
                 rawChartMode:
                   type: boolean
                 predictionEnabled:
@@ -6267,6 +6323,27 @@ components:
                 - WATERY
                 - EGG_WHITE
             - type: "null"
+        cervixPosition:
+          anyOf:
+            - type: string
+              enum:
+                - LOW
+                - HIGH
+            - type: "null"
+        cervixFirmness:
+          anyOf:
+            - type: string
+              enum:
+                - FIRM
+                - SOFT
+            - type: "null"
+        cervixOpening:
+          anyOf:
+            - type: string
+              enum:
+                - CLOSED
+                - OPEN
+            - type: "null"
         sexualActivity:
           type: boolean
         protectedSex:
@@ -6344,6 +6421,9 @@ components:
         - temperatureExcluded
         - ovulationTest
         - cervicalMucus
+        - cervixPosition
+        - cervixFirmness
+        - cervixOpening
         - sexualActivity
         - protectedSex
         - pregnancyTest
@@ -6677,6 +6757,27 @@ components:
                           - WATERY
                           - EGG_WHITE
                       - type: "null"
+                  cervixPosition:
+                    anyOf:
+                      - type: string
+                        enum:
+                          - LOW
+                          - HIGH
+                      - type: "null"
+                  cervixFirmness:
+                    anyOf:
+                      - type: string
+                        enum:
+                          - FIRM
+                          - SOFT
+                      - type: "null"
+                  cervixOpening:
+                    anyOf:
+                      - type: string
+                        enum:
+                          - CLOSED
+                          - OPEN
+                      - type: "null"
                 required:
                   - date
                   - phase
@@ -6691,6 +6792,9 @@ components:
                   - temperatureExcluded
                   - ovulationTest
                   - cervicalMucus
+                  - cervixPosition
+                  - cervixFirmness
+                  - cervixOpening
                 additionalProperties: false
             meta:
               type: object
@@ -7156,6 +7260,11 @@ components:
             - OFF
         cycleTrackingEnabled:
           type: boolean
+        secondarySymptom:
+          type: string
+          enum:
+            - MUCUS
+            - CERVIX
         rawChartMode:
           type: boolean
         predictionEnabled:
@@ -7189,6 +7298,7 @@ components:
       required:
         - goal
         - cycleTrackingEnabled
+        - secondarySymptom
         - rawChartMode
         - predictionEnabled
         - discreetNotifications

prisma/migrations/0157_v1170_cycle_secondary_symptom/migration.sql

@@ -0,0 +1,29 @@
+-- v1.17.0 — symptothermal SECONDARY-symptom choice: cervical mucus (default)
+-- OR cervix observation, mirroring the Sensiplan/NFP double-check where the
+-- temperature primary sign pairs with one chosen second indicator.
+--
+-- `cycle_profiles.secondary_symptom` picks which second sign the engine uses
+-- (MUCUS default | CERVIX); the casual user keeps temperature + mucus and never
+-- sees the choice (it lives behind the advanced cycle settings).
+--
+-- `cycle_day_logs` gains the three standard Sensiplan cervix signs — position
+-- (LOW/HIGH), firmness (FIRM/SOFT), opening/os (CLOSED/OPEN) — used as the
+-- secondary indicator when secondary_symptom = CERVIX. Fertile = HIGH+SOFT+OPEN;
+-- infertile (closure) = LOW+FIRM+CLOSED.
+--
+-- Additive + back-compatible: every existing profile backfills MUCUS (the prior
+-- behaviour) and every existing day-log row keeps NULL cervix signs. iOS
+-- inherits the fields without a migration of its own.
+
+CREATE TYPE "secondary_symptom" AS ENUM ('MUCUS', 'CERVIX');
+CREATE TYPE "cervix_position" AS ENUM ('LOW', 'HIGH');
+CREATE TYPE "cervix_firmness" AS ENUM ('FIRM', 'SOFT');
+CREATE TYPE "cervix_opening" AS ENUM ('CLOSED', 'OPEN');
+
+ALTER TABLE "cycle_profiles"
+  ADD COLUMN "secondary_symptom" "secondary_symptom" NOT NULL DEFAULT 'MUCUS';
+
+ALTER TABLE "cycle_day_logs"
+  ADD COLUMN "cervix_position" "cervix_position",
+  ADD COLUMN "cervix_firmness" "cervix_firmness",
+  ADD COLUMN "cervix_opening" "cervix_opening";

prisma/schema.prisma

@@ -3615,6 +3615,47 @@ enum CyclePhase {
   @@map("cycle_phase")
 }
 
+/// Which symptothermal SECONDARY sign the user observes alongside the
+/// temperature primary sign for the Sensiplan double-check. `MUCUS`
+/// (cervical-mucus quality) is the default; `CERVIX` swaps in the cervix
+/// observation (position / firmness / opening). The temperature primary
+/// sign is unchanged either way — only the second indicator of the
+/// double-check differs. Mirrors the `drip` app's Settings →
+/// Customization → Secondary symptom choice.
+enum SecondarySymptom {
+  MUCUS
+  CERVIX
+
+  @@map("secondary_symptom")
+}
+
+/// Cervix vertical position, a Sensiplan cervix sign. Fertile = `HIGH`
+/// (the cervix rises around ovulation); infertile = `LOW`.
+enum CervixPosition {
+  LOW
+  HIGH
+
+  @@map("cervix_position")
+}
+
+/// Cervix firmness, a Sensiplan cervix sign. Fertile = `SOFT` (the cervix
+/// softens around ovulation); infertile = `FIRM`.
+enum CervixFirmness {
+  FIRM
+  SOFT
+
+  @@map("cervix_firmness")
+}
+
+/// Cervix os / opening, a Sensiplan cervix sign. Fertile = `OPEN`;
+/// infertile = `CLOSED`.
+enum CervixOpening {
+  CLOSED
+  OPEN
+
+  @@map("cervix_opening")
+}
+
 /// Which method produced a cached CyclePrediction. `CALENDAR` is the
 /// median+MAD variability predictor; `SYMPTOTHERMAL` folds the
 /// sensiplan BBT + mucus confirmation; `TEMPERATURE_TREND` is the
@@ -3654,6 +3695,12 @@ model CycleProfile {
   typicalPeriodLength Int? @map("typical_period_length")
   lutealPhaseLength   Int? @map("luteal_phase_length")
 
+  /// Which symptothermal SECONDARY sign the user observes for the Sensiplan
+  /// double-check: `MUCUS` (default) or `CERVIX`. Lives behind the advanced
+  /// cycle settings — the casual user keeps temperature + mucus and never
+  /// sees the choice. The temperature primary sign is unchanged either way.
+  secondarySymptom SecondarySymptom @default(MUCUS) @map("secondary_symptom")
+
   /// When false, the app computes / stores NO predictions — pure logging
   /// (the Read-Your-Body posture for users who distrust algorithmic
   /// interpretation).
@@ -3761,6 +3808,16 @@ model CycleDayLog {
   /// Cervical-mucus quality.
   cervicalMucus  CervicalMucus? @map("cervical_mucus")
 
+  /// Cervix observation — the three standard Sensiplan cervix signs, used
+  /// as the symptothermal secondary indicator when
+  /// CycleProfile.secondarySymptom is CERVIX. All NULL = not observed.
+  /// The fertile combination is HIGH + SOFT + OPEN; the infertile
+  /// (closure) combination is LOW + FIRM + CLOSED. Queryable-plaintext like
+  /// the other physiology signs (mucus / BBT / OPK).
+  cervixPosition CervixPosition? @map("cervix_position")
+  cervixFirmness CervixFirmness? @map("cervix_firmness")
+  cervixOpening  CervixOpening?  @map("cervix_opening")
+
   /// Intent-revealing fields. When CycleProfile.sensitiveCategoryEncryption
   /// is ON these are encrypted at the application layer (the dispatch
   /// envelope), default-ON for this category.

src/app/api/auth/me/cycle-prefs/route.ts

@@ -92,6 +92,9 @@ export const PATCH = apiHandler(async (req: Request) => {
     data: {
       ...(p.enabled !== undefined && { cycleTrackingEnabled: p.enabled }),
       ...(p.goal !== undefined && { goal: p.goal }),
+      ...(p.secondarySymptom !== undefined && {
+        secondarySymptom: p.secondarySymptom,
+      }),
       ...(p.rawChartMode !== undefined && { rawChartMode: p.rawChartMode }),
       ...(p.predictionEnabled !== undefined && {
         predictionEnabled: p.predictionEnabled,

src/app/api/cycle/calendar/route.ts

@@ -102,6 +102,9 @@ export const GET = apiHandler(async (request: NextRequest) => {
         temperatureExcluded: true,
         ovulationTest: true,
         cervicalMucus: true,
+        cervixPosition: true,
+        cervixFirmness: true,
+        cervixOpening: true,
         _count: { select: { symptomLinks: true } },
       },
     }),
@@ -129,6 +132,9 @@ export const GET = apiHandler(async (request: NextRequest) => {
     temperatureExcluded: l.temperatureExcluded,
     ovulationTest: l.ovulationTest,
     cervicalMucus: l.cervicalMucus,
+    cervixPosition: l.cervixPosition,
+    cervixFirmness: l.cervixFirmness,
+    cervixOpening: l.cervixOpening,
     hasSymptoms: l._count.symptomLinks > 0,
   }));
 

src/app/api/cycle/insights/route.ts

@@ -93,6 +93,9 @@ export const GET = apiHandler(async () => {
         temperatureExcluded: true,
         ovulationTest: true,
         cervicalMucus: true,
+        cervixPosition: true,
+        cervixFirmness: true,
+        cervixOpening: true,
         symptomLinks: { select: { symptom: { select: { key: true } } } },
       },
     }),

src/lib/ai/coach/cycle-snapshot.ts

@@ -142,6 +142,7 @@ export async function buildCycleSnapshotBlock(
       lutealPhaseLength: true,
       typicalCycleLength: true,
       typicalPeriodLength: true,
+      secondarySymptom: true,
     },
   });
   if (!profile) return null;
@@ -153,6 +154,7 @@ export async function buildCycleSnapshotBlock(
     lutealPhaseLength: profile.lutealPhaseLength,
     predictionEnabled: profile.predictionEnabled,
     rawChartMode: profile.rawChartMode,
+    secondarySymptom: profile.secondarySymptom,
   };
 
   // Derive "today" from the user's timezone — matches how the calendar
@@ -179,6 +181,9 @@ export async function buildCycleSnapshotBlock(
           temperatureExcluded: true,
           ovulationTest: true,
           cervicalMucus: true,
+          cervixPosition: true,
+          cervixFirmness: true,
+          cervixOpening: true,
         },
       }),
       prisma.measurement.findMany({

src/lib/cycle/__tests__/engine-adapter.test.ts

@@ -55,7 +55,7 @@ describe("buildCalendar", () => {
 
   it("labels logged-flow days and emits a forecast for ≥2 cycles", () => {
     const dayLogs: CalendarDayLogRow[] = [
-      { date: "2026-02-26", flow: "MEDIUM", basalBodyTempC: null, temperatureExcluded: false, ovulationTest: null, cervicalMucus: null, hasSymptoms: true },
+      { date: "2026-02-26", flow: "MEDIUM", basalBodyTempC: null, temperatureExcluded: false, ovulationTest: null, cervicalMucus: null, cervixPosition: null, cervixFirmness: null, cervixOpening: null, hasSymptoms: true },
     ];
     const { prediction, days } = buildCalendar(
       profile(),

src/lib/cycle/__tests__/prediction.test.ts

@@ -4,6 +4,7 @@ import { addDays } from "../day-math";
 import {
   clampLuteal,
   confirmSymptothermal,
+  detectCervixPeak,
   detectLhSurgeOvulation,
   detectMucusPeak,
   detectTempShift,
@@ -297,6 +298,24 @@ describe("cycle/prediction — symptothermal confirmation (§4.2)", () => {
     return { date, flow: null, basalBodyTempC: t, ovulationTest: null, cervicalMucus: mucus };
   }
 
+  function cervixDay(
+    date: string,
+    cervixPosition: DayLogInput["cervixPosition"],
+    cervixFirmness: DayLogInput["cervixFirmness"],
+    cervixOpening: DayLogInput["cervixOpening"],
+  ): DayLogInput {
+    return {
+      date,
+      flow: null,
+      basalBodyTempC: null,
+      ovulationTest: null,
+      cervicalMucus: null,
+      cervixPosition,
+      cervixFirmness,
+      cervixOpening,
+    };
+  }
+
   it("detectTempShift: 3-over-6 with the 3rd reading >= 0.2°C over the 6-day max", () => {
     // 6 low baseline ~36.40, then 3 elevated clearing the max by 0.2.
     const logs: DayLogInput[] = [];
@@ -522,6 +541,115 @@ describe("cycle/prediction — symptothermal confirmation (§4.2)", () => {
     expect(confirmSymptothermal(logs)).toBe("2024-01-06");
   });
 
+  it("confirmSymptothermal defaults to the mucus secondary symptom (cervix ignored)", () => {
+    // A confirmed temp shift + a confirmed CERVIX peak but NO mucus must NOT
+    // confirm on the default (mucus) path — the default behaviour is unchanged.
+    const logs: DayLogInput[] = [];
+    const start = "2024-01-01";
+    [36.4, 36.42, 36.38, 36.41, 36.4, 36.39].forEach((t, i) =>
+      logs.push(bbtDay(addDays(start, i), t)),
+    );
+    logs.push(bbtDay(addDays(start, 6), 36.55));
+    logs.push(bbtDay(addDays(start, 7), 36.6));
+    logs.push(bbtDay(addDays(start, 8), 36.62));
+    // cervix peak on 2024-01-06, confirmed by 3 closed days — but no mucus.
+    logs.push(cervixDay("2024-01-06", "HIGH", "SOFT", "OPEN"));
+    logs.push(cervixDay("2024-01-07", "LOW", "FIRM", "CLOSED"));
+    logs.push(cervixDay("2024-01-08", "LOW", "FIRM", "CLOSED"));
+    logs.push(cervixDay("2024-01-09", "LOW", "FIRM", "CLOSED"));
+    // Default secondary = mucus → no mucus peak → no confirmation.
+    expect(confirmSymptothermal(logs)).toBeNull();
+  });
+
+  it("detectCervixPeak: confirms the last fertile-cervix day followed by 3 closed days", () => {
+    const start = "2024-01-01";
+    const logs = [
+      cervixDay(addDays(start, 0), "LOW", "FIRM", "CLOSED"),
+      cervixDay(addDays(start, 1), "HIGH", "SOFT", "OPEN"), // fertile peak
+      cervixDay(addDays(start, 2), "LOW", "FIRM", "CLOSED"),
+      cervixDay(addDays(start, 3), "LOW", "FIRM", "CLOSED"),
+      cervixDay(addDays(start, 4), "LOW", "FIRM", "CLOSED"),
+    ];
+    expect(detectCervixPeak(logs)).toBe(addDays(start, 1));
+  });
+
+  it("detectCervixPeak: returns null until 3 closed days follow the fertile peak", () => {
+    const start = "2024-01-01";
+    const logs = [
+      cervixDay(addDays(start, 0), "HIGH", "SOFT", "OPEN"),
+      cervixDay(addDays(start, 1), "LOW", "FIRM", "CLOSED"),
+      cervixDay(addDays(start, 2), "LOW", "FIRM", "CLOSED"),
+    ];
+    expect(detectCervixPeak(logs)).toBeNull();
+  });
+
+  it("confirmSymptothermal(CERVIX): a cervix peak + temp shift confirms like mucus does", () => {
+    const logs: DayLogInput[] = [];
+    const start = "2024-01-01";
+    [36.4, 36.42, 36.38, 36.41, 36.4, 36.39].forEach((t, i) =>
+      logs.push(bbtDay(addDays(start, i), t)),
+    );
+    logs.push(bbtDay(addDays(start, 6), 36.55));
+    logs.push(bbtDay(addDays(start, 7), 36.6));
+    logs.push(bbtDay(addDays(start, 8), 36.62));
+    // temp ovulation = 2024-01-06. Cervix peak on 2024-01-06, confirmed by 3
+    // closed days after it (cervix-closure rule).
+    logs.push(cervixDay("2024-01-06", "HIGH", "SOFT", "OPEN"));
+    logs.push(cervixDay("2024-01-07", "LOW", "FIRM", "CLOSED"));
+    logs.push(cervixDay("2024-01-08", "LOW", "FIRM", "CLOSED"));
+    logs.push(cervixDay("2024-01-09", "LOW", "FIRM", "CLOSED"));
+    expect(confirmSymptothermal(logs, "CERVIX")).toBe("2024-01-06");
+  });
+
+  it("confirmSymptothermal(CERVIX): a cervix sign alone (no temp shift) never confirms", () => {
+    const start = "2024-01-01";
+    // A confirmed cervix peak but a FLAT temperature series (no rise).
+    const logs: DayLogInput[] = [];
+    [36.4, 36.41, 36.39, 36.4, 36.42, 36.4, 36.41, 36.4, 36.39].forEach(
+      (t, i) => logs.push(bbtDay(addDays(start, i), t)),
+    );
+    logs.push(cervixDay(addDays(start, 4), "HIGH", "SOFT", "OPEN"));
+    logs.push(cervixDay(addDays(start, 5), "LOW", "FIRM", "CLOSED"));
+    logs.push(cervixDay(addDays(start, 6), "LOW", "FIRM", "CLOSED"));
+    logs.push(cervixDay(addDays(start, 7), "LOW", "FIRM", "CLOSED"));
+    // Single sign (cervix) without the temperature double-check → no confirmation.
+    expect(confirmSymptothermal(logs, "CERVIX")).toBeNull();
+  });
+
+  it("predictCycle(CERVIX): a cervix-peak + temp shift confirms the same boundary as mucus", () => {
+    const cervixProfile: CycleProfileInput = {
+      ...BASE_PROFILE,
+      secondarySymptom: "CERVIX",
+    };
+    const cycles = cyclesFromGaps("2024-01-01", [28, 28, 28]);
+    const lastStart = cycles[cycles.length - 1].startDate;
+    const logs: DayLogInput[] = [];
+    const ovDay = addDays(lastStart, 13);
+    const riseStart = addDays(ovDay, 1);
+    const baselineStart = addDays(riseStart, -6);
+    [36.4, 36.41, 36.39, 36.4, 36.42, 36.4].forEach((t, i) =>
+      logs.push(bbtDay(addDays(baselineStart, i), t)),
+    );
+    logs.push(bbtDay(riseStart, 36.6));
+    logs.push(bbtDay(addDays(riseStart, 1), 36.62));
+    logs.push(bbtDay(addDays(riseStart, 2), 36.63));
+    // cervix peak agreeing with the temp ovulation, confirmed by 3 closed days.
+    logs.push(cervixDay(ovDay, "HIGH", "SOFT", "OPEN"));
+    logs.push(cervixDay(addDays(ovDay, 1), "LOW", "FIRM", "CLOSED"));
+    logs.push(cervixDay(addDays(ovDay, 2), "LOW", "FIRM", "CLOSED"));
+    logs.push(cervixDay(addDays(ovDay, 3), "LOW", "FIRM", "CLOSED"));
+
+    const result = predictCycle(
+      cycles,
+      logs,
+      cervixProfile,
+      addDays(lastStart, 16),
+    );
+    expect(result.ovulationConfirmed).toBe(true);
+    expect(result.method).toBe("BLENDED");
+    expect(result.predictedOvulation).toBe(ovDay);
+  });
+
   it("predictCycle: a confirmed symptothermal ovulation overrides next-start and tightens the band", () => {
     const cycles = cyclesFromGaps("2024-01-01", [28, 28, 28]); // 3 lengths, last start 2024-03-25
     const lastStart = cycles[cycles.length - 1].startDate;

src/lib/cycle/backup.ts

@@ -21,6 +21,7 @@ export interface CycleBackupSection {
     typicalCycleLength: number | null;
     typicalPeriodLength: number | null;
     lutealPhaseLength: number | null;
+    secondarySymptom: string;
     predictionEnabled: boolean;
     rawChartMode: boolean;
     discreetNotifications: boolean;
@@ -43,6 +44,9 @@ export interface CycleBackupSection {
     temperatureExcluded: boolean;
     ovulationTest: string | null;
     cervicalMucus: string | null;
+    cervixPosition: string | null;
+    cervixFirmness: string | null;
+    cervixOpening: string | null;
     sexualActivity: boolean;
     protectedSex: boolean | null;
     pregnancyTest: string | null;
@@ -89,6 +93,7 @@ export async function buildCycleBackupSection(
           typicalCycleLength: profile.typicalCycleLength,
           typicalPeriodLength: profile.typicalPeriodLength,
           lutealPhaseLength: profile.lutealPhaseLength,
+          secondarySymptom: profile.secondarySymptom,
           predictionEnabled: profile.predictionEnabled,
           rawChartMode: profile.rawChartMode,
           discreetNotifications: profile.discreetNotifications,
@@ -112,6 +117,9 @@ export async function buildCycleBackupSection(
       temperatureExcluded: d.temperatureExcluded,
       ovulationTest: d.ovulationTest,
       cervicalMucus: d.cervicalMucus,
+      cervixPosition: d.cervixPosition,
+      cervixFirmness: d.cervixFirmness,
+      cervixOpening: d.cervixOpening,
       sexualActivity: d.sexualActivity,
       protectedSex: d.protectedSex,
       pregnancyTest: d.pregnancyTest,
@@ -156,6 +164,10 @@ const CERVICAL_MUCUS = new Set([
   "WATERY",
   "EGG_WHITE",
 ]);
+const SECONDARY_SYMPTOMS = new Set(["MUCUS", "CERVIX"]);
+const CERVIX_POSITIONS = new Set(["LOW", "HIGH"]);
+const CERVIX_FIRMNESSES = new Set(["FIRM", "SOFT"]);
+const CERVIX_OPENINGS = new Set(["CLOSED", "OPEN"]);
 const HOME_TESTS = new Set(["NEGATIVE", "POSITIVE", "INDETERMINATE"]);
 const CONTRACEPTIVES = new Set([
   "NONE",
@@ -221,6 +233,8 @@ export async function restoreCycleData(
         typicalCycleLength: p.typicalCycleLength ?? null,
         typicalPeriodLength: p.typicalPeriodLength ?? null,
         lutealPhaseLength: p.lutealPhaseLength ?? null,
+        secondarySymptom: (enumOrNull(p.secondarySymptom, SECONDARY_SYMPTOMS) ??
+          "MUCUS") as never,
         predictionEnabled: p.predictionEnabled ?? true,
         rawChartMode: p.rawChartMode ?? false,
         discreetNotifications: p.discreetNotifications ?? false,
@@ -294,6 +308,15 @@ export async function restoreCycleData(
         temperatureExcluded: d.temperatureExcluded ?? false,
         ovulationTest: enumOrNull(d.ovulationTest, OVULATION_TESTS) as never,
         cervicalMucus: enumOrNull(d.cervicalMucus, CERVICAL_MUCUS) as never,
+        cervixPosition: enumOrNull(
+          d.cervixPosition,
+          CERVIX_POSITIONS,
+        ) as never,
+        cervixFirmness: enumOrNull(
+          d.cervixFirmness,
+          CERVIX_FIRMNESSES,
+        ) as never,
+        cervixOpening: enumOrNull(d.cervixOpening, CERVIX_OPENINGS) as never,
         sexualActivity: d.sexualActivity ?? false,
         protectedSex: d.protectedSex ?? null,
         pregnancyTest: enumOrNull(d.pregnancyTest, HOME_TESTS) as never,

src/lib/cycle/day-log-write.ts

@@ -143,6 +143,9 @@ type ExistingRow = {
   temperatureExcluded: boolean;
   ovulationTest: string | null;
   cervicalMucus: string | null;
+  cervixPosition: string | null;
+  cervixFirmness: string | null;
+  cervixOpening: string | null;
   sexualActivity: boolean;
   protectedSex: boolean | null;
   pregnancyTest: string | null;
@@ -162,6 +165,9 @@ const EXISTING_SELECT = {
   temperatureExcluded: true,
   ovulationTest: true,
   cervicalMucus: true,
+  cervixPosition: true,
+  cervixFirmness: true,
+  cervixOpening: true,
   sexualActivity: true,
   protectedSex: true,
   pregnancyTest: true,
@@ -323,6 +329,18 @@ async function writeDayLog(args: WriteArgs): Promise<DayLogWriteResult> {
     entry.cervicalMucus !== undefined
       ? (entry.cervicalMucus ?? null)
       : (existing?.cervicalMucus ?? null);
+  const cervixPosition =
+    entry.cervixPosition !== undefined
+      ? (entry.cervixPosition ?? null)
+      : (existing?.cervixPosition ?? null);
+  const cervixFirmness =
+    entry.cervixFirmness !== undefined
+      ? (entry.cervixFirmness ?? null)
+      : (existing?.cervixFirmness ?? null);
+  const cervixOpening =
+    entry.cervixOpening !== undefined
+      ? (entry.cervixOpening ?? null)
+      : (existing?.cervixOpening ?? null);
 
   // Split the sensitive fields between plaintext columns and the envelope
   // depending on the flag. When encrypting, the plaintext columns are NULL
@@ -347,6 +365,9 @@ async function writeDayLog(args: WriteArgs): Promise<DayLogWriteResult> {
     temperatureExcluded,
     ovulationTest: ovulationTest as never,
     cervicalMucus: cervicalMucus as never,
+    cervixPosition: cervixPosition as never,
+    cervixFirmness: cervixFirmness as never,
+    cervixOpening: cervixOpening as never,
     sexualActivity: sensitivePlaintext.sexualActivity,
     protectedSex: sensitivePlaintext.protectedSex,
     pregnancyTest: sensitivePlaintext.pregnancyTest as never,
@@ -367,6 +388,9 @@ async function writeDayLog(args: WriteArgs): Promise<DayLogWriteResult> {
       existing.temperatureExcluded !== temperatureExcluded ||
       (existing.ovulationTest ?? null) !== ovulationTest ||
       (existing.cervicalMucus ?? null) !== cervicalMucus ||
+      (existing.cervixPosition ?? null) !== cervixPosition ||
+      (existing.cervixFirmness ?? null) !== cervixFirmness ||
+      (existing.cervixOpening ?? null) !== cervixOpening ||
       storedSensitive.sexualActivity !== resolvedSensitive.sexualActivity ||
       storedSensitive.protectedSex !== resolvedSensitive.protectedSex ||
       storedSensitive.pregnancyTest !== resolvedSensitive.pregnancyTest ||
@@ -406,6 +430,9 @@ async function writeDayLog(args: WriteArgs): Promise<DayLogWriteResult> {
         temperatureExcluded: baseData.temperatureExcluded as never,
         ovulationTest: baseData.ovulationTest as never,
         cervicalMucus: baseData.cervicalMucus as never,
+        cervixPosition: baseData.cervixPosition as never,
+        cervixFirmness: baseData.cervixFirmness as never,
+        cervixOpening: baseData.cervixOpening as never,
         sexualActivity: sensitivePlaintext.sexualActivity,
         protectedSex: sensitivePlaintext.protectedSex,
         pregnancyTest: sensitivePlaintext.pregnancyTest as never,

src/lib/cycle/dto.ts

@@ -33,6 +33,10 @@ export interface CycleDayLogDTO {
   temperatureExcluded: boolean;
   ovulationTest: string | null;
   cervicalMucus: string | null;
+  /** Cervix observation signs (symptothermal secondary indicator). */
+  cervixPosition: string | null;
+  cervixFirmness: string | null;
+  cervixOpening: string | null;
   sexualActivity: boolean;
   protectedSex: boolean | null;
   pregnancyTest: string | null;
@@ -112,6 +116,9 @@ export function toCycleDayLogDTO(row: DayLogWithLinks): CycleDayLogDTO {
     temperatureExcluded: row.temperatureExcluded,
     ovulationTest: row.ovulationTest,
     cervicalMucus: row.cervicalMucus,
+    cervixPosition: row.cervixPosition,
+    cervixFirmness: row.cervixFirmness,
+    cervixOpening: row.cervixOpening,
     sexualActivity: hasEnvelope
       ? (enc.sexualActivity ?? false)
       : row.sexualActivity,
@@ -206,6 +213,8 @@ export function toCyclePredictionDTO(
 export interface CycleProfileDTO {
   goal: string;
   cycleTrackingEnabled: boolean;
+  /** Symptothermal secondary symptom — MUCUS (default) or CERVIX. */
+  secondarySymptom: string;
   rawChartMode: boolean;
   predictionEnabled: boolean;
   discreetNotifications: boolean;
@@ -223,6 +232,7 @@ export function toCycleProfileDTO(
   return {
     goal: row.goal,
     cycleTrackingEnabled,
+    secondarySymptom: row.secondarySymptom,
     rawChartMode: row.rawChartMode,
     predictionEnabled: row.predictionEnabled,
     discreetNotifications: row.discreetNotifications,
@@ -236,6 +246,7 @@ export function toCycleProfileDTO(
 
 type MenstrualCycleProfileRow = {
   goal: string;
+  secondarySymptom: string;
   rawChartMode: boolean;
   predictionEnabled: boolean;
   discreetNotifications: boolean;

src/lib/cycle/engine-adapter.ts

@@ -54,6 +54,9 @@ export type CalendarDayLogRow = Pick<
   | "temperatureExcluded"
   | "ovulationTest"
   | "cervicalMucus"
+  | "cervixPosition"
+  | "cervixFirmness"
+  | "cervixOpening"
 > & { hasSymptoms: boolean };
 
 /** Map MenstrualCycle rows (oldest→newest) to engine `CycleInput`. */
@@ -86,6 +89,9 @@ export function toDayLogInputs(
     | "temperatureExcluded"
     | "ovulationTest"
     | "cervicalMucus"
+    | "cervixPosition"
+    | "cervixFirmness"
+    | "cervixOpening"
   >[],
 ): DayLogInput[] {
   return logs.map((l) => ({
@@ -95,6 +101,9 @@ export function toDayLogInputs(
     temperatureExcluded: l.temperatureExcluded,
     ovulationTest: l.ovulationTest,
     cervicalMucus: l.cervicalMucus,
+    cervixPosition: l.cervixPosition,
+    cervixFirmness: l.cervixFirmness,
+    cervixOpening: l.cervixOpening,
   }));
 }
 
@@ -107,6 +116,7 @@ export function toProfileInput(profile: CycleProfile): CycleProfileInput {
     lutealPhaseLength: profile.lutealPhaseLength,
     predictionEnabled: profile.predictionEnabled,
     rawChartMode: profile.rawChartMode,
+    secondarySymptom: profile.secondarySymptom,
   };
 }
 

src/lib/cycle/prediction.ts

@@ -57,6 +57,7 @@ import {
   type CycleProfileInput,
   type DayLogInput,
   type NightlyTempInput,
+  type SecondarySymptom,
   type PredictionMethod,
 } from "./types";
 
@@ -427,16 +428,89 @@ export function detectMucusPeak(
 }
 
 /**
- * §4.2 — symptothermal CONFIRMATION: ovulation is confirmed only when the
- * temp-shift day and the mucus-peak agree within ±2 days. Returns the
- * temp-derived ovulation day when confirmed, else null.
+ * §4.2(b') — sensiplan cervix peak (Höhepunkt), the CERVIX-secondary-symptom
+ * analogue of `detectMucusPeak`. The cervix is a fertile sign while it is HIGH,
+ * SOFT, or OPEN (any of the three Sensiplan cervix signs in its fertile state);
+ * it is infertile only when fully closed — LOW + FIRM + CLOSED. The peak day is
+ * the LAST fertile-cervix day FOLLOWED by at least 3 consecutive closed
+ * (infertile) observed days (the cervix-closure rule). Like the mucus peak it is
+ * only confirmable retrospectively, so a stray late fertile-cervix entry not yet
+ * followed by 3 closed days cannot move an already-confirmed peak.
+ *
+ * A cervix "day" counts as observed when at least one of the three signs is
+ * logged (an unlogged sign is treated as its fertile/uncertain state so a
+ * partial entry never fabricates a closure). Returns the confirmed peak day, or
+ * null when no fertile-cervix day has yet been followed by 3 closed days.
+ *
+ * Citation: cervix observation as a Sensiplan secondary sign — the cervix opens,
+ * softens and rises around ovulation and closes, firms and lowers afterward;
+ * evaluation follows the same "last fertile day + 3 confirming days" shape as
+ * mucus. Arbeitsgruppe NFP / Raith-Paula & Frank-Herrmann; myNFP "Den
+ * Gebärmutterhals beobachten".
+ */
+export function detectCervixPeak(
+  dayLogs: readonly DayLogInput[],
+): string | null {
+  // Observed cervix days only (at least one sign logged), oldest→newest.
+  const observed = dayLogs
+    .filter(
+      (l) =>
+        l.cervixPosition != null ||
+        l.cervixFirmness != null ||
+        l.cervixOpening != null,
+    )
+    .map((l) => ({ date: l.date, closed: isCervixClosed(l) }))
+    .sort((a, b) => dayDiff(a.date, b.date));
+
+  // Walk every fertile-cervix day; a candidate peak is confirmed iff the next 3
+  // observed cervix days are all closed (infertile). Scan forward and keep the
+  // LATEST confirmed peak (a true later peak supersedes an earlier one).
+  let confirmedPeak: string | null = null;
+  for (let i = 0; i < observed.length; i++) {
+    if (observed[i].closed) continue; // only a fertile-cervix day can be a peak
+    const following = observed.slice(i + 1, i + 4);
+    if (following.length < 3) continue; // not yet evaluable
+    if (following.every((d) => d.closed)) confirmedPeak = observed[i].date;
+  }
+  return confirmedPeak;
+}
+
+/**
+ * A cervix is in its infertile (closed) state only when ALL three signs read
+ * infertile: LOW + FIRM + CLOSED. Any fertile sign (HIGH / SOFT / OPEN) — or an
+ * unlogged sign, treated conservatively as not-yet-closed — keeps the day
+ * fertile, so a partial entry never fabricates a closure.
+ */
+function isCervixClosed(log: DayLogInput): boolean {
+  return (
+    log.cervixPosition === "LOW" &&
+    log.cervixFirmness === "FIRM" &&
+    log.cervixOpening === "CLOSED"
+  );
+}
+
+/**
+ * §4.2 — symptothermal CONFIRMATION (the Sensiplan double-check): ovulation is
+ * confirmed only when the temperature primary sign AND the chosen SECONDARY sign
+ * both confirm and agree within ±2 days. The secondary sign is the cervical
+ * mucus peak by default, or the cervix peak when `secondarySymptom` is `CERVIX`.
+ * Returns the temp-derived ovulation day when confirmed, else null.
+ *
+ * A single sign never confirms — no temperature shift returns null regardless of
+ * the secondary sign, and no secondary peak returns null regardless of the
+ * temperature. This is the honesty principle: the double-check requires two
+ * indicators, never one.
  */
 export function confirmSymptothermal(
   dayLogs: readonly DayLogInput[],
+  secondarySymptom: SecondarySymptom = "MUCUS",
 ): string | null {
   const shift = detectTempShift(dayLogs, TEMP_SHIFT_C_MANUAL);
   if (!shift) return null;
-  const peak = detectMucusPeak(dayLogs);
+  const peak =
+    secondarySymptom === "CERVIX"
+      ? detectCervixPeak(dayLogs)
+      : detectMucusPeak(dayLogs);
   if (!peak) return null;
   if (
     Math.abs(dayDiff(shift.ovulationDate, peak)) <= SYMPTOTHERMAL_AGREE_DAYS
@@ -671,7 +745,10 @@ export function predictCycle(
     dayDiff(date, windowStart) >= 0 && dayDiff(today, date) >= 0;
   const currentDayLogs = dayLogs.filter((l) => isInCurrentWindow(l.date));
   const currentNights = nights.filter((n) => isInCurrentWindow(n.date));
-  const symptoOvulation = confirmSymptothermal(currentDayLogs);
+  const symptoOvulation = confirmSymptothermal(
+    currentDayLogs,
+    profile.secondarySymptom,
+  );
   const trendOvulation = symptoOvulation
     ? null
     : detectTemperatureTrend(currentNights);

src/lib/cycle/types.ts

@@ -34,6 +34,22 @@ export type CervicalMucus =
   | "WATERY"
   | "EGG_WHITE";
 
+/**
+ * Which symptothermal SECONDARY sign pairs with the temperature primary sign
+ * for the Sensiplan double-check. `MUCUS` (cervical-mucus quality) is the
+ * default; `CERVIX` swaps in the cervix observation.
+ */
+export type SecondarySymptom = "MUCUS" | "CERVIX";
+
+/** Cervix vertical position — fertile = HIGH, infertile = LOW. */
+export type CervixPosition = "LOW" | "HIGH";
+
+/** Cervix firmness — fertile = SOFT, infertile = FIRM. */
+export type CervixFirmness = "FIRM" | "SOFT";
+
+/** Cervix os / opening — fertile = OPEN, infertile = CLOSED. */
+export type CervixOpening = "CLOSED" | "OPEN";
+
 export type CyclePhase = "MENSTRUAL" | "FOLLICULAR" | "OVULATORY" | "LUTEAL";
 
 export type PredictionMethod =
@@ -215,6 +231,15 @@ export interface DayLogInput {
   temperatureExcluded?: boolean | null;
   ovulationTest: OvulationTest | null;
   cervicalMucus: CervicalMucus | null;
+  /**
+   * Cervix observation — the three Sensiplan cervix signs. Used as the
+   * symptothermal SECONDARY indicator when the profile's `secondarySymptom`
+   * is `CERVIX`; ignored on the default (mucus) path. All optional for
+   * back-compat; an unobserved sign is `null`/absent.
+   */
+  cervixPosition?: CervixPosition | null;
+  cervixFirmness?: CervixFirmness | null;
+  cervixOpening?: CervixOpening | null;
 }
 
 /**
@@ -240,6 +265,13 @@ export interface CycleProfileInput {
   predictionEnabled: boolean;
   /** Read-Your-Body mode: suppress all interpretation. */
   rawChartMode: boolean;
+  /**
+   * Which symptothermal SECONDARY sign the double-check uses — `MUCUS`
+   * (default) or `CERVIX`. Optional for back-compat; the engine treats an
+   * absent value as `MUCUS`, so existing callers keep the mucus path
+   * unchanged.
+   */
+  secondarySymptom?: SecondarySymptom;
 }
 
 /* ------------------------------------------------------------------ */

src/lib/openapi/routes/cycle.ts

@@ -11,6 +11,10 @@ import {
   flowLevelEnum,
   ovulationTestEnum,
   cervicalMucusEnum,
+  secondarySymptomEnum,
+  cervixPositionEnum,
+  cervixFirmnessEnum,
+  cervixOpeningEnum,
   homeTestResultEnum,
   cycleTrackingGoalEnum,
   cycleDayLogInputSchema,
@@ -104,6 +108,9 @@ const cycleDayLogDto = z
     temperatureExcluded: z.boolean(),
     ovulationTest: ovulationTestEnum.nullable(),
     cervicalMucus: cervicalMucusEnum.nullable(),
+    cervixPosition: cervixPositionEnum.nullable(),
+    cervixFirmness: cervixFirmnessEnum.nullable(),
+    cervixOpening: cervixOpeningEnum.nullable(),
     sexualActivity: z.boolean(),
     protectedSex: z.boolean().nullable(),
     pregnancyTest: homeTestResultEnum.nullable(),
@@ -179,12 +186,16 @@ const cycleCalendarDayDto = z.object({
   temperatureExcluded: z.boolean(),
   ovulationTest: ovulationTestEnum.nullable(),
   cervicalMucus: cervicalMucusEnum.nullable(),
+  cervixPosition: cervixPositionEnum.nullable(),
+  cervixFirmness: cervixFirmnessEnum.nullable(),
+  cervixOpening: cervixOpeningEnum.nullable(),
 });
 
 const cycleProfileDto = z
   .object({
     goal: cycleTrackingGoalEnum,
     cycleTrackingEnabled: z.boolean(),
+    secondarySymptom: secondarySymptomEnum,
     rawChartMode: z.boolean(),
     predictionEnabled: z.boolean(),
     discreetNotifications: z.boolean(),

src/lib/validations/backup.ts

@@ -144,6 +144,9 @@ const cycleDayLogSchema = z
     temperatureExcluded: z.boolean().optional(),
     ovulationTest: z.string().nullable().optional(),
     cervicalMucus: z.string().nullable().optional(),
+    cervixPosition: z.string().nullable().optional(),
+    cervixFirmness: z.string().nullable().optional(),
+    cervixOpening: z.string().nullable().optional(),
     sexualActivity: z.boolean().optional(),
     protectedSex: z.boolean().nullable().optional(),
     pregnancyTest: z.string().nullable().optional(),
@@ -166,6 +169,7 @@ const cycleProfileSchema = z
     typicalCycleLength: z.number().int().nullable().optional(),
     typicalPeriodLength: z.number().int().nullable().optional(),
     lutealPhaseLength: z.number().int().nullable().optional(),
+    secondarySymptom: z.string().nullable().optional(),
     predictionEnabled: z.boolean().optional(),
     rawChartMode: z.boolean().optional(),
     discreetNotifications: z.boolean().optional(),

src/lib/validations/cycle.ts

@@ -51,6 +51,14 @@ export const cervicalMucusEnum = z.enum([
   "EGG_WHITE",
 ]);
 
+/** Symptothermal secondary-symptom choice (mirrors the Prisma enum). */
+export const secondarySymptomEnum = z.enum(["MUCUS", "CERVIX"]);
+
+/** The three Sensiplan cervix signs (mirror the Prisma enums). */
+export const cervixPositionEnum = z.enum(["LOW", "HIGH"]);
+export const cervixFirmnessEnum = z.enum(["FIRM", "SOFT"]);
+export const cervixOpeningEnum = z.enum(["CLOSED", "OPEN"]);
+
 export const homeTestResultEnum = z.enum([
   "NEGATIVE",
   "POSITIVE",
@@ -108,6 +116,11 @@ export const cycleDayLogInputSchema = z.object({
   temperatureExcluded: z.boolean().optional(),
   ovulationTest: ovulationTestEnum.optional(),
   cervicalMucus: cervicalMucusEnum.optional(),
+  // Cervix observation — the symptothermal secondary indicator when the
+  // profile's secondarySymptom is CERVIX.
+  cervixPosition: cervixPositionEnum.optional(),
+  cervixFirmness: cervixFirmnessEnum.optional(),
+  cervixOpening: cervixOpeningEnum.optional(),
   sexualActivity: z.boolean().optional(),
   protectedSex: z.boolean().nullable().optional(),
   pregnancyTest: homeTestResultEnum.optional(),
@@ -138,6 +151,9 @@ export const cycleDayLogPatchSchema = z.object({
   temperatureExcluded: z.boolean().optional(),
   ovulationTest: ovulationTestEnum.nullable().optional(),
   cervicalMucus: cervicalMucusEnum.nullable().optional(),
+  cervixPosition: cervixPositionEnum.nullable().optional(),
+  cervixFirmness: cervixFirmnessEnum.nullable().optional(),
+  cervixOpening: cervixOpeningEnum.nullable().optional(),
   sexualActivity: z.boolean().optional(),
   protectedSex: z.boolean().nullable().optional(),
   pregnancyTest: homeTestResultEnum.nullable().optional(),
@@ -192,6 +208,9 @@ export const cyclePrefsSchema = z
   .object({
     enabled: z.boolean().optional(),
     goal: cycleTrackingGoalEnum.optional(),
+    // Symptothermal secondary symptom — advanced cycle setting (MUCUS default
+    // | CERVIX). Drives which second sign the double-check uses.
+    secondarySymptom: secondarySymptomEnum.optional(),
     rawChartMode: z.boolean().optional(),
     predictionEnabled: z.boolean().optional(),
     discreetNotifications: z.boolean().optional(),