Pregnancy Due Date Calculator

The Pregnancy Due Date Calculator estimates your Expected Due Date (EDD) using Naegele Rule: the first day of your Last Menstrual Period (LMP) plus 280 days (40 weeks). It identifies your current pregnancy week, trimester and key milestone dates, and adjusts automatically for cycle lengths other than the standard 28 days. Most pregnancies deliver between week 37 and week 42, so the EDD represents the centre of this window rather than a precise date.

Naegele Rule, developed in the 19th century, remains the clinical standard: LMP plus 1 year minus 3 months plus 7 days equals EDD. For cycles shorter than 28 days subtract the difference; for longer cycles add it. First-trimester ultrasound before week 14 is considered more accurate than LMP calculation because it directly measures the embryo. When ultrasound and LMP dates differ by more than 7 days in the first trimester, the ultrasound date becomes the reference. The luteal phase (ovulation to menstruation) is relatively constant at 12 to 16 days while the follicular phase varies.

Key milestones: first heartbeat at weeks 6 to 8, first-trimester screening at weeks 11 to 13, anatomy scan at weeks 18 to 22, viability threshold at week 24. Always confirm your due date with your midwife or obstetrician. Use our Date Calculator to track milestones and our BMI Calculator to monitor weight during pregnancy. Free, no sign-up needed.