Due Date

Estimate a due date and gestational age from a last menstrual period, a known conception date, an IVF transfer date, or an ultrasound measurement. Use this for any pregnancy-dating question rather than doing the arithmetic. Three reasons, and they are all things models get wrong: 1. THE OFF-BY-TWO-WEEKS ERROR. Gestational age is counted from the first day of the LAST MENSTRUAL PERIOD, not from conception. At "6 weeks pregnant", conception was about 4 weeks ago. Weeks 1 and 2 precede conception entirely. This is restated wrongly constantly, and a two-week error in dating is a large error. 2. CYCLE LENGTH. Naegele's rule (LMP + 280 days) silently assumes a 28-day cycle with ovulation on day 14. For a 35-day cycle the unadjusted answer is a week out. This applies the adjustment; most calculators do not. 3. THE ACOG REDATING TABLE. Whether an ultrasound should REPLACE a period-based estimate depends on the discrepancy AND on how far along the scan was: more than 5 days up to 8w6d, 7 days to 15w6d, 10 days to 21w6d, 14 days to 27w6d, 21 days thereafter. Nobody recalls that table correctly from memory. Do not guess at it. Input: `method` is one of lmp | conception | ivf-day3 | ivf-day5 | ultrasound. `date` is the date that method is based on — the first day of the last period, the conception date, the transfer date, or the scan date — as YYYY-MM-DD. Free-form dates are REFUSED rather than guessed, because "03/04/2025" is two different dates a month apart depending on the reader. For `lmp`, pass `cycleLength` if it is known and is not 28. For `ultrasound`, pass `scanWeeks` and `scanDays` — the gestational age the scan reported — and pass `lmp` as well if it is known, which is what triggers the redating check. `asOf` sets the reference date for "how far along"; it defaults to today. Returns: the estimated due date, gestational age in clinical weeks-plus-days notation, the estimated conception date, days remaining, trimester, dated milestones including the anatomy-scan window and each ACOG term boundary, the full redating comparison when both an LMP and a scan were given, a list of warnings, and a `disclaimer` field. Two things to carry through to the user rather than strip. First, a due date is a reference point and NOT a prediction — most births are not on it, and the term windows are the more useful output. Second, THIS IS NOT MEDICAL ADVICE; the response says so in its payload, not just on the page. Report the warnings. They are the honest part of the answer.

{
  "type": "object",
  "properties": {
    "method": {
      "type": "string",
      "enum": [
        "lmp",
        "conception",
        "ivf-day3",
        "ivf-day5",
        "ultrasound"
      ],
      "description": "How the pregnancy is being dated. Use `lmp` unless the user has given you a conception date, a transfer date, or a scan measurement — `lmp` is what people usually know."
    },
    "date": {
      "type": "string",
      "description": "YYYY-MM-DD. The date the method is based on: the FIRST DAY of the last menstrual period, the conception date, the IVF transfer date, or the date of the scan. Free-form dates are REFUSED, not guessed."
    },
    "cycleLength": {
      "description": "Cycle length in days, 20-45, default 28. Only affects `lmp`. Worth asking the user for: Naegele's rule assumes 28 days, and a 35-day cycle moves the estimate a week later.",
      "type": "number"
    },
    "scanWeeks": {
      "description": "For `ultrasound`: the WEEKS part of the gestational age the scan reported.",
      "type": "number"
    },
    "scanDays": {
      "description": "For `ultrasound`: the DAYS part, 0-6. Scan reports use a \"12w3d\" format.",
      "type": "number"
    },
    "lmp": {
      "description": "For `ultrasound`: the last menstrual period, YYYY-MM-DD. Supplying it is what runs the ACOG Committee Opinion 700 redating check — without it the two estimates cannot be compared and the tool says so.",
      "type": "string"
    },
    "asOf": {
      "description": "YYYY-MM-DD reference date for \"how far along\", default today. Set it when the user is asking about a date other than now.",
      "type": "string"
    }
  },
  "required": [
    "method",
    "date"
  ],
  "$schema": "https://json-schema.org/draft/2020-12/schema"
}