How the due date is calculated
Pregnancy is counted from the first day of the last menstrual period (LMP), not from conception. With a regular 28-day cycle, ovulation and conception happen about two weeks after the LMP, so at conception you are already "2 weeks pregnant". A full-term pregnancy is counted as 40 weeks, or 280 days, from the LMP.
Adding 280 days to the LMP is known as Naegele's rule. If your cycles are longer than 28 days, you probably ovulated later, so the due date moves later by the same number of days. Shorter cycles move it earlier. For IVF, the embryo's age at transfer is already known, so 3 or 5 days are taken off the 266 days from conception.
Worked example
The first day of the last period was 1 January 2026 and cycles are 28 days long. 1 January 2026 + 280 days = 8 October 2026. With a 32-day cycle, add 4 more days: the due date becomes 12 October 2026. A conception date of 15 January 2026 (+266 days), a day-3 transfer on 18 January (+263) or a day-5 transfer on 20 January (+261) all give the same 8 October due date.
On 9 April 2026, 98 days after that LMP, the pregnancy is 14 weeks and 0 days, the first day of the second trimester.
Weeks pregnant and trimesters
The gestational age is the number of whole weeks and days since the LMP (or the equivalent date for the other methods). "12 weeks and 3 days" means 12 full weeks plus 3 days. This calculator follows the American College of Obstetricians and Gynecologists (ACOG) trimester split:
| Stage | Gestational age | From an LMP of 1 Jan 2026 |
|---|---|---|
| First trimester | 0 weeks to the end of week 13 (13w 6d) | 1 Jan – 8 Apr 2026 |
| Second trimester | 14w 0d to 27w 6d | 9 Apr – 15 Jul 2026 |
| Third trimester | 28w 0d until birth | From 16 Jul 2026 |
| Early term | 37w 0d to 38w 6d | From 17 Sep 2026 |
| Full term | 39w 0d to 40w 6d | From 1 Oct 2026 |
| Late term | 41w 0d to 41w 6d | From 15 Oct 2026 |
| Post-term | 42w 0d and later | From 22 Oct 2026 |
Scan windows in the key dates table (a dating scan at about 8–14 weeks and an anomaly scan at about 18–22 weeks) are typical ranges. The exact timing of scans and tests depends on your country and your care provider.
How accurate is a due date?
A due date is an estimate, not a deadline. Only about 4–5% of babies are born on their due date, and most are born within two weeks either side of it. Cycle lengths vary, ovulation does not always happen on the same day, and many people are not sure of the exact date of their last period.
That is why a dating ultrasound in early pregnancy is the more accurate method. Measuring the baby in the first trimester gives a reliable estimate of gestational age. If the scan date and the period date differ, your clinician will usually go by the scan. Once your clinician gives you a due date, use that one.
Which method should I use?
- Last period: the most common starting point. Choose your usual cycle length if it is not 28 days.
- Conception date: use it only if you know when you conceived, for example from ovulation tracking.
- IVF transfer: the most precise date-based method, because the embryo's age is known. Choose day 3 or day 5.
Trying to conceive? The ovulation calculator estimates your fertile days. To count the days between any two dates, use the date calculator.
This calculator gives general information only. It is not medical advice and does not replace care from a midwife, doctor or obstetrician.
Frequently asked questions
How is a due date calculated?
How many weeks pregnant am I?
When does the second trimester start?
How do I calculate an IVF due date?
Will my baby arrive on the due date?
Is the ultrasound due date more accurate?
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