Pregnancy Due Date Calculator

Naegele’s rule adds 280 days to the first day of the last menstrual period, a formula in clinical use since 1812. It gives the right answer for a 28-day cycle. For every day the actual cycle differs from 28, the estimated due date shifts by exactly one day. A 32-day cycle moves the EDD four days later; a 24-day cycle moves it four days earlier. This calculator applies that adjustment automatically, so the date it returns reflects the individual cycle rather than the textbook average.

Three dating methods cover the full range of real inputs. Most people arrive with either the LMP date from memory or a gestational age figure from a scan report. Ultrasound dating is the most accurate approach before 14 weeks, because first-trimester crown-rump length measurements carry a margin of only ±5–7 days against the ±2-week uncertainty of cycle recall.

How to Use Each Dating Option

1. Last Menstrual Period (default). Enter the first day of your LMP. Press Load Sample to prefill 2026-07-01 with a 28-day cycle, which gives an EDD of 7 April 2027 and a gestational age of 13 weeks 0 days as of 29 September 2026. Drag the cycle-length slider (20–45 days) if your cycle is not 28 days; the EDD updates immediately.
2. Conception Date. Enter the date of conception. The tool subtracts 14 days (the standard ovulation offset) to recover the equivalent LMP and then applies Naegele from there.
3. Ultrasound. Enter the scan date and the gestational age printed on the report (weeks plus days). The LMP is back-calculated by subtracting that number of days from the scan date, then the EDD follows from LMP + 280. Cycle-length correction is bypassed because the fetal measurement already encodes when ovulation occurred.

How the Formula Works

The core formula is EDD = LMP + 280 + (cycleLength − 28) days. For the ultrasound method the equivalent is LMP = scanDate − gestationalDays, then EDD = LMP + 280. The milestone timeline marks seven checkpoints: end of the first trimester at week 12, anatomy scan window at week 18, WHO viability threshold at week 24 (168 days), third trimester at week 27, ACOG full term at week 39, the due date at week 40, and the post-term boundary 14 days after the EDD. Past checkpoints appear struck-through. Entering an LMP more than 300 days ago returns “LMP date is more than 300 days ago — please verify.”

Accuracy Limits and When to Rely on Ultrasound

Cycle-recall dating carries an inherent ±2-week uncertainty because most people cannot pinpoint ovulation precisely. Ultrasound measurements taken before 14 weeks narrow that range to ±5–7 days. After 14 weeks, individual fetal growth rates diverge and scan-based redating loses reliability; the first-trimester CRL measurement is what clinicians treat as definitive. This tool performs the same arithmetic a midwife or OB runs manually and is a planning aid. Clinical decisions, including whether to revise the EDD after a later scan, belong with a qualified healthcare provider.