Pregnancy Due Date Calculator

Naegele's rule and the cycle adjustment

The classic method adds 280 days to the first day of your last period. That works out to nine months plus seven days and assumes textbook 28-day cycles. This calculator adjusts for your actual cycle length: a 32-day cycle, for example, pushes the due date about four days later because ovulation happens later.

Why an ultrasound matters

Last-period dating carries real uncertainty — irregular cycles, spotting mistaken for a period, and simple recall error all shift it. A dating scan in the first trimester measures the embryo directly and becomes the official due date if it differs from this estimate by more than about a week.

Worked example

Last period started 1 January, cycle length 28 days. Due date = 1 January + 280 days = 8 October. Estimated conception is around 15 January (day 14). The second trimester starts about 2 April (13 weeks) and the third about 9 July (27 weeks). With a 32-day cycle the due date shifts to about 12 October, but the trimester dates stay put.

What the milestones mean

• Estimated conception — roughly when fertilisation occurred, about two weeks after the last period on a typical cycle.
• How far along now — gestational age in weeks and days, counted from the last period.
• Trimester starts — the conventional 13- and 27-week marks from the last period.
• Due date — 40 weeks. Only about 4% of babies arrive on it; most come within two weeks either side.

This is an estimate, not medical advice. Confirm dates with your midwife or doctor, and use a first-trimester ultrasound for the accurate date.