Calculate estimated due date, trimester timeline, and key milestones from LMP or conception.
Enter the first day of your last menstrual period (LMP) or an estimated conception date and the tool returns an estimated due date (EDD), how far along the pregnancy is in weeks and days, and the calendar dates each trimester starts. It applies the standard dating conventions: 280 days (40 weeks) from the LMP, or 266 days from conception, with trimester boundaries at the 14- and 28-week marks. What surprises most users: the due date is an estimate, not a prediction. Only about 1 in 20 babies arrives on the exact date, and a pregnancy is considered term from 37 weeks 0 days through 41 weeks 6 days.
A concrete input and expected output from the current implementation.
Input
Last menstrual period: January 1, 2026
Expected output
Estimated due date: October 8, 2026 (40 weeks from LMP). Estimated conception: January 15, 2026. First trimester: January 1 to April 8, 2026. Second trimester: April 9 to July 15, 2026. Third trimester: July 16 to October 8, 2026.
The tool counts 280 days from the LMP, the standard convention, which places ovulation and estimated conception 14 days after the period. Each trimester boundary then falls at the conventional 14-week and 28-week marks, which are days 98 and 196 of the pregnancy.
The LMP path counts forward 280 days, or 40 weeks, to the estimated due date, because the standard dating convention treats the pregnancy as starting on the first day of the period and assumes ovulation 14 days later. Gestational age is the elapsed time since the LMP, expressed in completed weeks and days, such as 24 weeks 3 days.
The conception path counts forward 266 days, or 38 weeks, which is the 280-day LMP convention minus the assumed 14 days between the period and ovulation. The two paths agree when the dates entered are consistent with each other. This path fits pregnancies where the ovulation date is actually known, for example from temperature or hormone tracking, and it avoids the cycle-length assumption entirely.
Probably not, and that is normal. The estimated due date marks 40 weeks, but a pregnancy is considered term from 37 weeks 0 days through 41 weeks 6 days, and only about 1 in 20 babies is born on the exact date. Treat the date as a planning anchor for scans, tests, and leave, not as a prediction of the birth day.
The 280-day count assumes ovulation on day 14 of a 28-day cycle. With longer cycles, ovulation typically happens later, so the actual due date tends to be later than this estimate, and earlier with shorter cycles. If you know your ovulation date, use the conception-based calculation, which does not assume a cycle length. A first-trimester ultrasound is the most accurate dating method, and your provider may set a different date.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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