Due Date Calculator

The most common due-date formula adds 280 days to the first day of your last menstrual period (LMP) for a 28-day cycle. For other cycle lengths, clinicians adjust: EDD = LMP + 280 days + (cycle length − 28). Only about 5% of babies arrive on the exact EDD.

Ultrasound dating in the first trimester can refine the estimate — especially with irregular cycles. This tool performs calendar math only; your obstetric care team sets the official due date.

Example: LMP January 10, 2026 with a 28-day cycle → EDD October 17, 2026. A 32-day cycle pushes the EDD four days later to October 21, 2026.

The 280-day rule is named after Franz Naegele and assumes two things that are often untrue: a 28-day cycle and ovulation on day 14. Cycle length varies between people and between months, and ovulation timing varies even within a regular cycle — which is why the same LMP can support due dates several days apart.

Clinical guidance treats first-trimester ultrasound as the more reliable measure. The American College of Obstetricians and Gynecologists recommends dating by crown-rump length when an early scan differs from LMP dating by more than a set number of days, because measurement error is smallest before 14 weeks. Once an official date is set, it is generally not revised by later scans.

“Full term” is a range rather than a date. ACOG defines early term as 37 weeks 0 days to 38 weeks 6 days, full term as 39 weeks 0 days to 40 weeks 6 days, late term as week 41, and post-term from 42 weeks. Most spontaneous births fall within a couple of weeks either side of the estimated date.

The due date does more than mark an end point: it anchors the whole schedule of care. Screening windows, the anatomy scan around 18 to 22 weeks, glucose testing near 24 to 28 weeks, and decisions about induction are all timed from it, which is why a shift of a few days in early dating can move several appointments.

Cycle-length adjustment helps only when your cycles are genuinely predictable. With irregular cycles, PCOS, recent hormonal contraception, or breastfeeding, LMP-based math can be off by a week or more — an early ultrasound is the practical answer. This tool performs calendar math only, and your obstetric care team sets the official date.

Only about 5% of babies arrive on the exact EDD — most births fall within two weeks before or after. Use the date for planning, not as a precise appointment for labor onset.