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Pregnancy due date calculator

Find your estimated due date from the first day of your last period, your conception date or an IVF transfer date — plus how many weeks pregnant you are, your trimester and the key dates ahead.

  • Free
  • No sign-up
  • Private — runs in your browser

How do I calculate my due date?

  1. 1Choose how to work it out: last period, conception date or IVF transfer.
  2. 2Enter the date — and your usual cycle length if you use your last period.
  3. 3Read your estimated due date, how far along you are today and your trimester.
  4. 4Check the key dates, such as the end of the first trimester, the anatomy scan window and full term.

How is a due date worked out?

  1. 01Due date = first day of last period + 280 days (+ cycle length − 28)
  2. 02Due date = conception date + 266 days
  3. 03IVF: due date = transfer date + 261 days (day 5) or + 263 days (day 3)
  4. 04Weeks pregnant = days since (due date − 280 days) ÷ 7

Pregnancy is counted from the first day of the last menstrual period, not from conception, and lasts about 40 weeks (280 days). This is Naegele's rule, used by doctors and midwives worldwide. It assumes a 28-day cycle with ovulation on day 14; if your cycle is longer or shorter, ovulation usually moves too, so the due date moves by the same number of days.

If you know the conception date, the due date is 266 days (38 weeks) later. For IVF the embryo's age is known exactly: a day-5 blastocyst transfer is 19 days after the "period" date doctors count from, so the due date is 261 days after transfer; a day-3 transfer is 263 days after.

How far along you are is the time since that counting start date, in weeks and days. An early ultrasound (before about 14 weeks) is the most accurate way to date a pregnancy, and doctors usually change the due date if the scan and period dates differ by more than a few days.

Good to know

  • Use the first day of bleeding of your last period, not the last day.
  • If your cycles are irregular, the date from an early ultrasound scan is more reliable than this estimate.
  • Most babies are born between 37 and 42 weeks — plan for a window, not a single day.
  • Write down your scan dates: the anatomy scan is usually booked between 18 and 21 weeks (NHS) or 18 and 22 weeks.

Due date calculator — common questions

How is my due date calculated?

By adding 280 days (40 weeks) to the first day of your last period, adjusted for your cycle length, or 266 days to the conception date. Your doctor or midwife may adjust it after an ultrasound scan.

How many weeks pregnant am I?

Pregnancy is counted from the first day of your last period. The calculator shows your weeks and days from that date to today — so at conception you are already about 2 weeks pregnant.

How accurate is a due date?

It is an estimate. Only about 1 in 20 babies is born on the due date itself; most arrive in the weeks around it. An early ultrasound gives the most accurate date.

How do I work out my due date after IVF?

Add 261 days to the transfer date for a day-5 blastocyst, or 263 days for a day-3 embryo. Choose "IVF transfer" and the embryo age above.

When do the trimesters start?

This calculator uses the common split: first trimester to the end of week 13, second trimester weeks 14 to 27, third trimester from week 28 until birth.

What do early term, full term and post-term mean?

Following ACOG: early term is 37 weeks to 38 weeks 6 days, full term 39 weeks to 40 weeks 6 days, late term 41 weeks, and post-term from 42 weeks.

Sources

Formulas last reviewed: September 30, 2026 ·How we check our formulas

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