How it works
Doctors date a pregnancy from the first day of the last menstrual period (LMP), not from conception, because the LMP is usually the date people know. A typical pregnancy lasts about 280 days (40 weeks) from the LMP. This is known as Naegele’s rule.
due date = LMP + 280 days + (cycle length − 28 days)The standard rule assumes a 28-day cycle with ovulation around day 14. If your cycles are longer or shorter, ovulation usually happens later or earlier, so the calculator moves the due date by the difference. If you know the conception date (for example after fertility treatment), choose “conception date”: the due date is then conception + 266 days.
Worked example. LMP on 1 January 2026 with a 28-day cycle: 1 January + 280 days = Thursday 8 October 2026. Estimated conception is around 15 January 2026. On 12 March 2026 the pregnancy is 10 weeks and 0 days, in the first trimester. With a 32-day cycle the due date moves 4 days later, to 12 October 2026.
Trimesters
| Trimester | Weeks |
|---|---|
| First | 0 to 13 weeks + 6 days |
| Second | 14 to 27 weeks + 6 days |
| Third | 28 weeks to birth |
How accurate is a due date?
Only about 4–5% of babies are born on the exact due date. Most are born within two weeks either side of it, and a birth from 37 weeks is considered full term. An early ultrasound (before about 14 weeks), which measures the baby, is more accurate than any date-based calculation, and your doctor may adjust your due date after it. This calculator is an estimate and does not replace antenatal care.
The due date is also shown on the Hijri calendar (Umm al-Qura), which many families in the Gulf use. To plan a pregnancy, see the ovulation calculator; to count days to any appointment, use days from today.