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PregnancyHow the due date is calculated
The most common method — Naegele's rule — adds 280 days (40 weeks) to the first day of the last menstrual period. This is based on the assumption of a standard 28-day menstrual cycle with ovulation on day 14.
Three calculation methods
- Last menstrual period (LMP): the most common method, used early in pregnancy when the exact conception date is unknown.
- Conception date: if the exact conception date is known (e.g., with IVF), 266 days (38 weeks) are added.
- Ultrasound dating: the most accurate method in early pregnancy, since it's based on the actual size of the fetus rather than cycle assumptions.
Pregnancy trimesters
- First trimester: weeks 1-13
- Second trimester: weeks 14-27
- Third trimester: weeks 28-40+
Why the date is only an estimate
Statistically, only 4-5% of babies are born exactly on the calculated day. Most births occur between 37 and 42 weeks of pregnancy, so the calculated date is more of a midpoint of the expected range than a precise prediction.
This calculator provides an estimate and doesn't replace medical consultation. For accurate determination of gestational age and due date, consult an OB/GYN.
FAQ
How is the due date calculated?
The most common method (Naegele's rule) adds 280 days (40 weeks) to the first day of the last menstrual period.
How accurate is this date?
This is an estimated date — only 4-5% of babies are born exactly on the calculated day. Most births occur within 2 weeks before or after this date.
What if my cycle is irregular?
With an irregular cycle, the LMP-based calculation may be inaccurate. In this case, a doctor typically relies on the first ultrasound, which more accurately determines gestational age.