Pregnancy due date calculator

Classic Naegele assumes a twenty-eight-day cycle: add one year, subtract three months, and add seven days. Missing LMP or a cycle length that is not a positive number of days fails with invalid_input instead of inventing a date.

What cycle-adjusted Naegele's rule computes from LMP and cycle length

Naegele's rule estimates the expected date of delivery from the first day of the last menstrual period under the simplifying assumption of ovulation near the middle of a regular cycle. In the form still printed in textbooks and clinic posters, you take that LMP day, add one full year, subtract three months, and add seven days, then normalize any day that overflows the length of the resulting month. A thirty-day cycle therefore adds two days after the classic Naegele date; a twenty-one-day cycle subtracts seven. That calendar transform is the twenty-eight-day baseline this endpoint implements byte for byte. Real cycles are often not twenty-eight days, so the second required field cycle_length, a whole positive number of days from the first day of one period to the first day of the next, shifts the due date by cycle_length minus twenty-eight civil days. Required fields are lmp, an ISO calendar date written as YYYY-MM-DD naming the first bleeding day of the last period, and cycle_length. The response also echoes the normalized lmp and cycle_length, labels method as naegele_cycle, restates formula as EDD equals LMP plus one year minus three months plus seven days plus cycle_length minus twenty-eight, reports gestation_days as the whole civil day count between LMP and EDD, and reports cycle_adjustment_days so a UI can show how far the result moved from the twenty-eight-day baseline. 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

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 or time zones to this capability; this page is the textbook Naegele LMP-to-EDD transform with an explicit cycle-length offset only. POST a JSON object whose required field lmp is a string matching the strict pattern four digits, hyphen, two-digit month, hyphen, two-digit day, and whose required field cycle_length is a positive whole number of days. Aliases last_menstrual_period, lmp_date, and period_start map to lmp; aliases cycle_days and cycle map to cycle_length, so forms and integrations can keep their preferred names. Missing lmp, null, empty string, non-string objects, and strings that fail the pattern all throw invalid_input naming the field and the expected shape. Cycle length must be a finite integer from one through ninety inclusive: zero, negatives, fractions, NaN, and empty strings are not a positive number of days and fail with invalid_input naming cycle_length. Those validation failures are not billed, so you can probe hostile or incomplete payloads without spending prepaid balance. On success the body always includes lmp, cycle_length, due_date, method, formula, gestation_days, and cycle_adjustment_days; optional null fields are never padded in. Example: lmp two thousand twenty-six January first with cycle_length twenty-eight yields due_date two thousand twenty-six October eighth under classic Naegele with a zero-day cycle adjustment. The same LMP with cycle_length thirty yields two days later.

Where a cycle-aware pregnancy due date API earns its keep

Pregnancy intake forms and maternity booking flows often need a first-pass estimated due date the moment a patient types her LMP and usual cycle length, before any ultrasound file is attached. Prefer ultrasound-based dating when clinical policy requires it, and treat Naegele output as the conventional calendar estimate it has always been: simple, explainable, and deterministic once cycle length is stated. Embedding this calculator gives a transparent Naegele baseline with a published formula and a cycle_adjustment_days field that nurses and product copy can quote without reverse-engineering a black-box date library.