Pregnancy Due Date Calculator

The standard medical method for estimating a due date is Naegele's rule, named after the German obstetrician Franz Karl Naegele (1778–1851). It assumes a 28-day menstrual cycle with ovulation on day 14, and predicts that pregnancy lasts 280 days from the first day of the last menstrual period (LMP):

EDD = LMP + 280 days

Two equivalent routes give the same answer. The conception-based route assumes ovulation 14 days after LMP, then adds 266 days post-conception:

EDD = conception date + 266 days

For cycles longer or shorter than 28 days, ovulation moves with the cycle, so the calculator shifts EDD by cycle − 28 days. A 30-day cycle pushes EDD two days later; a 26-day cycle pulls it two days earlier. This adjustment is consistent with the Mittendorf et al. (1990) analysis of cycle length and gestation duration.

Ultrasound dating between 7w0d and 13w6d uses crown–rump length (CRL) to measure gestational age directly. ACOG Committee Opinion No. 700 recommends replacing the LMP-based EDD with the ultrasound-based estimate whenever the two disagree by more than 5 days in the first trimester. This calculator applies that rule of thumb in the ultrasound tab: it back-calculates the effective LMP from the scan and uses that for all subsequent dates.

Trimester boundaries follow the modern ACOG convention: the first trimester spans 0w0d through 13w6d (gestational days 0–97), the second from 14w0d through 27w6d (days 98–195), and the third from 28w0d onward (day 196 and beyond). Some older sources split at week 12 and week 24; we use the more common ACOG version because it lines up cleanly with prenatal screening schedules.

All date arithmetic runs in UTC so the result does not drift across timezones, and the calculator emits an exact day count rather than relying on the classical month-arithmetic form ("subtract three months, add seven days, add one year"), which produces small discrepancies near month-end dates. Both routes — LMP-based and conception-based — are exposed in the data module so the answer can be cross-checked.

• ACOG Committee Opinion No. 700 — Methods for Estimating the Due Date (2017, reaffirmed 2022)
• NICE NG201 — Antenatal care (2024 revision)
• WHO — Antenatal care for a positive pregnancy experience (2016)
• Mittendorf R. et al. (1990) — The length of uncomplicated human gestation

The dating algorithm, trimester boundaries, and milestone weeks on this page were last cross-checked against the sources above on 2026-05-11. This tool is not a substitute for advice from a qualified obstetrician.