Pregnancy Due Date Calculator

Pregnancy due dates are estimated from the first day of the last menstrual period (LMP) using Naegele's rule: due date = LMP + 280 days (40 weeks). This assumes a 28-day menstrual cycle with ovulation on day 14. Healthcare providers use the same rule for dating pregnancies before ultrasound measurement refines the estimate. Only about 4–5% of babies arrive on the exact due date; most are born between 38 and 42 weeks.

Example: if the first day of the last period was March 1, add 280 days. March has 31 days, so March 1 + 30 days = March 31, plus 250 more days lands on December 6 (counting through April, May, June, July, August, September, October, November). That December date is the estimated due date — gestational age at that point is 40 weeks 0 days. Today at April 1 would be 4 weeks 5 days gestational age (31 days from March 1).

Ultrasound dating in the first trimester (especially 8–13 weeks) is more accurate than LMP alone when cycle length varies or ovulation is late. If ultrasound differs from LMP by more than 5–7 days in early pregnancy, clinicians typically adjust the due date to match crown-rump length measurements. Later ultrasounds are less reliable for changing the due date.

Gestational age is counted from LMP, not conception. Conception typically occurs about two weeks after LMP in a 28-day cycle, so at 8 weeks gestational age the embryo is roughly 6 weeks old biologically. This convention avoids confusion in clinical records — always specify "gestational weeks" when discussing pregnancy timelines.

Enter the first day of your last menstrual period to calculate the estimated due date, current gestational week, and days remaining. This is a planning estimate — attend prenatal appointments for ultrasound confirmation and follow your provider's guidance on the official expected delivery date.