Pregnancy Due Date Calculator
Enter the first day of your last menstrual period (LMP) and this calculator estimates the due date using Naegele's rule: 280 days, or 40 weeks, from the LMP. If your cycle is longer or shorter than the standard 28 days, the calculator shifts the estimate accordingly, since ovulation happens later in longer cycles.
It also shows how many weeks pregnant you are today and the start of each trimester. Keep in mind that a due date is a statistical estimate, and only about 1 in 20 babies arrives on the exact date. Most are born within two weeks either side, and your provider may revise the date after an early ultrasound.
How it's calculated
Due date = LMP + 280 days + (cycle length − 28)
The 280 days assume ovulation on day 14 of a 28-day cycle. A 32-day cycle typically means ovulation around day 18, so 4 days are added. Pregnancy weeks count from the LMP, which is why the count includes about two weeks before conception.
Worked example
With an LMP of 10 January 2026 and a 28-day cycle, the due date is 17 October 2026. On 30 August 2026 that pregnancy is at 33 weeks and 1 day. With a 33-day cycle instead, the due date shifts 5 days later to 22 October 2026.
Frequently asked questions
›How accurate is a due date?
About 4 to 5% of babies arrive on their due date. Roughly two thirds are born within a week of it, and delivery any time from 37 to 42 weeks is considered normal term. First-trimester ultrasound dating is more accurate than LMP dating when the two disagree.
›When does each trimester start?
The most common convention: the first trimester covers weeks 1 to 13, the second weeks 14 to 27, and the third from week 28 to birth. Sources vary by a week in either direction.
›What if I don't know my LMP?
An early ultrasound can date the pregnancy from the embryo's size, usually within a few days of accuracy. If you know your conception date, add 266 days instead of using the LMP.
›Do irregular cycles change the estimate?
Yes, LMP-based dating assumes predictable ovulation, so irregular cycles make it less reliable. In that case providers lean on ultrasound dating. The cycle length field here helps only with consistently longer or shorter cycles.
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