Estimate pregnancy due date

When the LMP is after today, or either date is missing or not a real calendar day, the task fails with invalid_input instead of inventing a schedule.

What Naegele's 280-day rule computes and why both lmp and today are required

Naegele's rule estimates the expected date of delivery from the first day of the last menstrual period under the conventional forty-week obstetric count from LMP. In the day-count form used by this capability, you take that LMP day and add exactly two hundred eighty whole civil days, normalizing month lengths and Gregorian leap years along the way. That fixed offset matches the classic bedside teaching of forty weeks from the first bleeding day of the last period for a regular twenty-eight-day cycle with ovulation near day fourteen. Two required fields drive the solve path. The field lmp is an ISO calendar date written as YYYY-MM-DD naming the first bleeding day of the last period the patient reports. This endpoint implements that arithmetic byte for byte: it does not call Date.now, it does not depend on the server timezone, and it does not silently adjust for cycle length. If lmp falls strictly after today, the request is rejected as invalid_input with a message that names both dates, because a last menstrual period in the future relative to the stated reference is not a usable baseline for due-date estimation. The response also echoes the normalized lmp and today, labels method as naegele_280, restates formula as EDD equals LMP plus two hundred eighty days, reports gestation_days as two hundred eighty, and reports days_since_lmp as the whole civil day count from LMP to the stated today so a UI can show gestational progress without re-deriving the serial math. This is a dating estimate only: it is not a diagnosis, not a guarantee of delivery on that day, and not a substitute for clinical ultrasound dating when that is available.

Input contract, validation, response shape, and deterministic errors

The field lmp must match the strict pattern four digits, hyphen, two-digit month, hyphen, two-digit day. Month must fall between one and twelve; day must be real for that month, so values such as two thousand twenty-six February thirtieth are rejected because February never has thirty days, and two thousand twenty-three February twenty-ninth is rejected because that year is not a leap year under the Gregorian rule. Do not send wall-clock timestamps, time zones, or cycle length to this capability—those belong to other tools; this page is intentionally the fixed two-hundred-eighty-day LMP-to-EDD transform with an explicit today guard. POST a JSON object with two required string fields. Aliases last_menstrual_period, lmp_date, and period_start map to the same parse path so forms can keep their preferred names. The field today must use the same ISO shape; aliases as_of and reference_date are accepted. Missing lmp or today, null, empty string, non-string objects, and strings that fail the pattern all throw invalid_input naming the field and the expected shape. On success the body always includes lmp, today, due_date, method, formula, gestation_days, and days_since_lmp; optional null fields are never padded in. Example: lmp two thousand twenty-six January first with today two thousand twenty-six March fifteenth yields due_date two thousand twenty-six October eighth under the fixed two-hundred-eighty-day offset, with method naegele_280 and days_since_lmp equal to the civil span from January first to March fifteenth. Failed validation is not billed, so you can probe hostile or incomplete payloads without spending prepaid balance.

Where an estimate pregnancy due date API earns its keep in product and education

Pregnancy intake forms and maternity booking flows often need a first-pass estimated due date the moment a patient types her LMP, with a clear rule that rejects future-dated periods relative to the form's reference day. Embedding this calculator gives a transparent Naegele baseline with a published formula and a days-since-LMP field that product copy can quote without reverse-engineering a black-box date library. Prefer ultrasound-based dating when clinical policy requires it, and treat the two-hundred-eighty-day output as the conventional calendar estimate it has always been: simple, explainable, and deterministic.