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', '$schema': 'https://json-schema.org/draft/2020-12/schema', 'required': ['method', 'date'], 'properties': {'lmp': {'type': 'string', 'description': 'For `ultrasound`: the last menstrual period, YYYY-MM-DD. Supplying it is what runs the ACOG Committee Opinion 700 redating check â\x80\x94 without it the two estimates cannot be compared and the tool says so.'}, 'asOf': {'type': 'string', '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.'}, '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.'}, 'method': {'enum': ['lmp', 'conception', 'ivf-day3', 'ivf-day5', 'ultrasound'], 'type': 'string', '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 â\x80\x94 `lmp` is what people usually know.'}, 'scanDays': {'type': 'number', 'description': 'For `ultrasound`: the DAYS part, 0-6. Scan reports use a "12w3d" format.'}, 'scanWeeks': {'type': 'number', 'description': 'For `ultrasound`: the WEEKS part of the gestational age the scan reported.'}, 'cycleLength': {'type': 'number', '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."}}}