Due Date Calculator

Estimate your baby's due date from your last period, conception date or IVF transfer, and see how many weeks pregnant you are today.

Reviewed by Dany, RightSums team · Updated

Runs in your browser. Free, no sign-up.

health · Daily life · UK & US

Enter the first day of your last period and your usual cycle length, or a conception date, or an IVF embryo transfer date. The calculator adds 280 days to your last period, moving the date one day for each day your cycle is longer or shorter than 28, as the NHS calculator does. From conception it adds 266 days, and after IVF it adds 261 days for a day-5 embryo or 263 for a day-3 embryo, following ACOG. You see your due date, how many weeks and days pregnant you are, your trimester and key dates such as scan windows. It is an estimate, not medical advice. Your dating scan may change the date, and only about 4.4% of babies arrive on it.

Accepted inputs

  • First day of last period
  • Average cycle length (20 to 44 days)
  • Conception date
  • IVF embryo transfer date and embryo age (day 3 or day 5)

Outputs

  • Estimated due date
  • Weeks and days pregnant today
  • Current trimester
  • Key dates: scan windows, trimesters, 37 and 42 weeks

How to use the Due Date Calculator

  1. Choose whether to count from your last period, a conception date or an IVF embryo transfer date, then enter the date.
  2. For the last period method, enter your usual cycle length and the calculator moves the standard 280-day count by the difference from 28 days.
  3. It works out your due date and counts the weeks and days from the start of the pregnancy to today, which gives your trimester.
  4. A table then lists key dates, including the dating scan window, the start of the third trimester and 37 to 42 weeks.

Frequently asked questions

How is my due date calculated?

Your due date is 280 days, or 40 weeks, after the first day of your last period. If your last period started on 10 January 2026, your estimated due date is 17 October 2026. On 1 May 2026 you would be 15 weeks and 6 days pregnant. This assumes a 28-day cycle, so a dating scan may move the date.

Does cycle length change my due date?

Yes. The NHS calculator moves the date one day later for each day your cycle is longer than 28 days, and one day earlier for each day shorter. With a last period starting 10 January 2026 and a 32-day cycle, the due date moves from 17 October to 21 October 2026. If your cycle is irregular, a scan is the better guide.

How do I work out my due date after IVF?

Count from the embryo transfer date: add 261 days for a day-5 embryo or 263 days for a day-3 embryo, as ACOG recommends. A day-5 transfer on 2 February 2026 gives a due date of 21 October 2026; a day-3 transfer that day gives 23 October 2026. If your clinic gave you a due date, use theirs.

Can my due date change after the 12-week scan?

Yes. The dating scan, usually at 10 to 14 weeks, measures the baby and is used to work out your due date, so it can replace the date from your last period. ACOG advises changing the date if an early scan differs by more than 7 days. Only about 4.4% of babies arrive on their due date, and 37 to 42 weeks is normal.

What weeks are the three trimesters?

The NHS guide puts the 1st trimester up to week 12, the 2nd from week 13 to 27 and the 3rd from week 28. ACOG in the US ends the 1st trimester at 13 weeks 6 days. With a last period on 10 January 2026, the 3rd trimester starts on 25 July 2026. Speak to a midwife, GP or doctor about anything that worries you.

Sources

The rates and rules in this tool come from:

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