Pregnancy Due Date Calculator

Enter the first day of your last period to see your estimated due date and how far along you are.

First day of the last period, or the day of conception

Results

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Pregnancy is counted from the first day of the last menstrual period and lasts 40 weeks — 280 days — on average. That is why “weeks pregnant” runs about two weeks ahead of the actual date of conception.

The calculator uses Naegele’s rule and shifts the date if your cycle is longer or shorter than 28 days. If you know the conception date (after IVF, for example) you can enter that instead.

How to calculate Due date

  1. Choose whether to calculate from your last period or from conception.
  2. Enter the date and your average cycle length.
  3. Read the estimated due date, how many weeks pregnant you are and when each trimester starts.

Due date formula

  • Due date = first day of last period + 280 days + (cycle length − 28)
  • From conception: due date = conception date + 266 days
  • Weeks pregnant = days since the last period ÷ 7

Due date example

With a last period starting on 1 January 2026 and a 28-day cycle, the estimated due date is 8 October 2026. On 1 April 2026 that is 12 weeks and 6 days pregnant.

Due date: common questions

Will the baby arrive on that date?

Usually not. Only about 4–5% of babies are born on their due date; most arrive between weeks 38 and 42.

What if the ultrasound gives a different date?

A first-trimester ultrasound is generally more reliable. If your doctor has set a date, use that one.

What if my periods are irregular?

With irregular cycles a calculation from the last period can be misleading; dating is then done by ultrasound.