What it does
This calculator estimates when a baby is due. It works from the first day of your last period, from a known conception date, or from an IVF embryo transfer date. It also shows how many weeks and days pregnant you are on a chosen date, which trimester that is, and the dates when the main weeks begin, such as the usual scan weeks and full term.
It is an estimate, not medical advice. Your dating scan is more accurate, so go by the scan when your care team gives you a date. To plan earlier, the ovulation calendar estimates fertile days, and the sleep calculator helps with bedtimes.
How to use it
- Choose what you know in Work it out from.
- Enter that date. For a period date, set your usual cycle length.
- Read the due date, the week count and the table of key weeks.
What each setting does
- Work it out from: last period (Naegele’s rule, +280 days), conception (+266 days), or IVF transfer of a day-3 (+263) or day-5 (+261) embryo.
- Usual cycle length: only for a period date. Each day longer than 28 moves the due date one day later, each day shorter one day earlier. Lengths outside 20 to 45 days fall back to 28.
- Today’s date: filled in from your device; change it to see the week count on another day.
- Reset: back to the example.
Weeks are always counted from two weeks before conception, the way doctors and midwives count them, whichever method you use.
Worked example
The last period started on 1 January 2026 and cycles are 28 days. 280 days later is Thursday 8 October 2026. With a 35-day cycle, ovulation is about a week later, so the due date moves to 15 October 2026. For a day-5 embryo transferred on 15 January 2026, the due date is 15 January + 261 days = 3 October 2026. On 1 March 2026, the first example is 8 weeks 3 days, first trimester.
FAQs
How exact is the due date? Only about 1 baby in 25 is born on it. Most arrive within two weeks either side.
Why do weeks start before conception? Pregnancy is counted from the last period, which is about two weeks before ovulation.
My scan gave a different date. Which is right? Use the scan date.
Where the numbers come from
- Naegele's rule (first day of last period + 280 days), with the cycle-length adjustment used in obstetric practice; conception + 266 days (checked 2026-11-10).
- American College of Obstetricians and Gynecologists, Committee Opinion 700 (2017), Methods for Estimating the Due Date: ultrasound in the first trimester is the most accurate method; IVF due date = transfer + 266 - embryo age, giving +263 for day-3 and +261 for day-5 embryos (checked 2026-11-10).
- ACOG Committee Opinion 579 (2013), Definition of Term Pregnancy: early term from 37 weeks, post-term from 42 weeks (checked 2026-11-10).
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Read more on the Blog
- How a pregnancy due date is worked out: periods, cycles and IVF(the full guide)
Last updated 2026-11-10.