health.gestational-age

export function gestationalAge(lmpDate: string, onDate: string, cycleLengthDays: number): GestationalAge

/** Gestational age as weeks+days, and the estimated due date. */
export interface GestationalAge {
  /** completed weeks */
  readonly weeks: number;
  /** days beyond the completed weeks, 0 to 6 */
  readonly days: number;
  /** gestational age in days */
  readonly totalDays: number;
  /** estimated due date, 40+0 weeks */
  readonly dueDate: string;
}

import { gestationalAge } from "#fune/health.gestational-age@^1";

import { addDays, daysBetween, parseIsoDate } from "./dates_add_days.ts";  ← from dates.add-days ^1.0.0 · built alongside by fune
import { type GestationalAge } from "./health_gestational_age_types.ts";

/** Naegele's rule: 280 days (40+0 weeks) from the LMP for a 28-day cycle. */
const TERM_DAYS = 280;
/** 44+0 weeks: beyond this the dates are almost certainly wrong. */
const MAX_DAYS = 308;

/**
 * Gestational age and estimated due date from the last menstrual period. A
 * cycle longer or shorter than 28 days moves ovulation, so both are shifted by
 * the difference, as the NHS due date calculator does.
 */
export function gestationalAge(lmpDate: string, onDate: string, cycleLengthDays: number): GestationalAge {
  if (!Number.isInteger(cycleLengthDays) || cycleLengthDays < 20 || cycleLengthDays > 44) {
    throw new RangeError(`cycleLengthDays must be a whole number from 20 to 44, received ${cycleLengthDays}`);
  }
  parseIsoDate(lmpDate);
  parseIsoDate(onDate);
  const shift = cycleLengthDays - 28;
  const totalDays = daysBetween(lmpDate, onDate) - shift;
  if (totalDays < 0) {
    throw new RangeError(`onDate ${onDate} is before the adjusted start of pregnancy`);
  }
  if (totalDays > MAX_DAYS) {
    throw new RangeError(`gestational age of ${totalDays} days is more than 44+0 weeks: check the dates`);
  }
  return {
    weeks: Math.floor(totalDays / 7),
    days: totalDays % 7,
    totalDays,
    dueDate: addDays(lmpDate, TERM_DAYS + shift),
  };
}

Install

fune build

With that line in your source, in a TypeScript project (language typescript in fune.project), fune build resolves it and its 1 dependency, pins them in fune.lock, downloads only the TypeScript package of each, and builds the code above into your project’s .fune/build, one readable file per capability with a header linking back here. Or pin a range in fune.project and build in one step:

fune add health.gestational-age

The manifest, vectors and README with only the TypeScript implementation. Install it without the registry with fune add ./health.gestational-age-1.0.0-typescript.fune, or fetch it from a terminal with fune pull health.gestational-age@1.0.0:typescript.

The whole function, every language, is one file too: health.gestational-age-1.0.0.fune, 14,669 bytes, sha256 eece582e3c18dfa4333d0a6782552ad603158f9d4cd53dcbddbfcbc2caec00d3. It installs into a project of any language.

// fune: before health.gestational-age

// fune: after health.gestational-age

// fune: replace dates.add-days in health.gestational-age

// fune: step health.gestational-age after <n|label>

More from the author

**The rule.** Naegele's rule puts the due date 280 days (40+0 weeks) after the LMP, which assumes a 28-day cycle with ovulation on day 14. A longer or shorter cycle moves ovulation, so both the due date and the gestational age are shifted by the difference from 28 days:

dueDate = LMP + 280 + (cycleLengthDays - 28) totalDays = (onDate - LMP) - (cycleLengthDays - 28) weeks = totalDays div 7, days = totalDays mod 7

With a 35-day cycle the due date is a week later and the pregnancy is a week less advanced on any date. Pass 28 for the unadjusted rule. This is exactly what the NHS due date calculator does, and its published answers are among the vectors (LMP 15 March 2026: due 20 December 2026 on a 28-day cycle, 27 December on 35 days, 12 December on 20 days, 5 January 2027 on 44 days).

**Limits, and why they are errors.**

- `cycleLengthDays` must be a whole number from 20 to 44, the range the NHS calculator accepts. - A measuring date before the adjusted start of pregnancy (the LMP plus the cycle shift) is an error, not a negative age: with a 35-day cycle, three days after the LMP is not yet 0+0. - A gestational age above 44+0 weeks (308 days) is an error asking for the dates to be checked. Pregnancy normally lasts 37 to 42 weeks from the LMP; 44 weeks is a guard rail against a wrong year or a transposed date, not a clinical threshold, and a clinician should confirm it.

Dates are ISO `YYYY-MM-DD`; the measuring date is always an argument, never today's date. Leap days are counted like any other day.

## Sources

- NHS, "Due date calculator" (nhs.uk): takes the first day of the last period and a cycle length that "must be between 20 and 44 days"; pregnancy "normally lasts from 37 weeks to 42 weeks from the first day of your last period". Its outputs for the dates above were checked on 23 September 2026. https://www.nhs.uk/pregnancy/finding-out/due-date-calculator/ - NICE guideline NG201, "Antenatal care" (2021), recommendation 1.2.14: offer an ultrasound scan between 11+2 and 14+1 weeks to determine gestational age. https://www.nice.org.uk/guidance/ng201/chapter/Recommendations

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