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Due date calculator
The due date is an estimate: enter the first day of your last menstrual period (LMP) and the tool applies Naegele's rule, adding 280 days, or 40 weeks. It also shows how many weeks pregnant you are today. Remember it is a reference, not a promise: very few babies are born exactly on the calculated date.
Pregnancy milestones from the LMP
| Milestone | Weeks | Approximately |
|---|---|---|
| End of first trimester | 13 weeks | LMP + 91 days |
| Halfway point | 20 weeks | LMP + 140 days |
| End of second trimester | 27 weeks | LMP + 189 days |
| Full term (start) | 37 weeks | LMP + 259 days |
| Estimated due date | 40 weeks | LMP + 280 days |
Naegele's rule
Estimated due date = LMP + 280 days (40 weeks)
The classic rule can also be written as taking the LMP, going back three months and adding seven days. For an LMP on 1 January 2025, the 280 days lead to 8 October 2025. For an LMP on 14 February 2025, the due date falls on 21 November 2025. Both counts assume a regular 28 day cycle, with ovulation around day 14. Gestational age is always counted from the LMP, not from conception, which is why pregnancy is described as 40 weeks even though fertilization happens about two weeks after the period begins.
Why it is only an estimate
It is estimated that only about 5% of babies are born exactly on the due date; most arrive within a two week window around it. Irregular cycles, ovulation on different days and uncertain period dates all shift the calculation. That is why a first trimester ultrasound, which measures the embryo, is usually more precise and may correct the date. To understand your cycle better, see the ovulation calculator. Source of the rule and clinical guidance: American College of Obstetricians and Gynecologists (ACOG).
Disclaimer: an educational estimate by Naegele's rule. It does not replace prenatal care or ultrasound. Follow your pregnancy with your obstetrician.
Last updated: · Methodology and sources