Calculators

Pregnancy Due Date Calculator

Your estimated due date, how many weeks along you are and your trimester, from your last period or conception date.

How it works

Naegele's rule counts 40 weeks from the first day of your last menstrual period. That assumes ovulation on day 14 of a 28-day cycle, so a longer cycle moves the date later by the difference and a shorter one earlier. From a known conception date the pregnancy lasts 266 days, 38 weeks. Weeks of pregnancy are always counted from the period, which is why you are already 2 weeks pregnant at conception.

due date = last period + 280 days + (cycle length − 28)
LMP
First day of the last menstrual period
First trimester
Weeks 1 to 13
Second trimester
Weeks 14 to 27
Third trimester
Week 28 to birth

Worked example

A last period that started on 10 February 2026, with a 28-day cycle.

  1. 110 February 2026 + 280 days = 17 November 2026
  2. 2Likely conception, 14 days after the period started: 24 February 2026
  3. 3The second trimester begins at 14 weeks: 19 May 2026
  4. 4The anatomy scan window, 18–22 weeks: 16 June to 14 July 2026

The estimated due date is 17 November 2026, and full term (37 weeks) begins on 27 October 2026.

Frequently asked questions

How accurate is a due date?

Only about 4% of babies arrive on the day itself. Around 80% are born within two weeks either side of it, and anything from 37 to 42 weeks counts as term. An early ultrasound narrows the estimate to within about 5–7 days.

What if my cycles are irregular?

Enter your typical cycle length if it is fairly steady. If cycles vary a lot, or you do not remember the date of your last period, the dating scan is the reliable guide, and your provider will set the date from it.

Why am I counted as 2 weeks pregnant at conception?

Pregnancy weeks are counted from the first day of the last period, about two weeks before ovulation, because that date is usually known and conception is not. So 40 weeks of pregnancy is really about 38 weeks from conception.

What about IVF pregnancies?

Use the IVF due date calculator. With IVF the embryo's age at transfer is known exactly, so the date is worked out from the transfer rather than from a period.

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