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Due date calculator
Due date = first day of the last menstrual period + 280 days, with the current gestational age and trimester boundaries computed from the same date.
These are estimates for planning, built from published methods (Naegele, ACOG, IOM) — not antenatal care. Your clinician adjusts dates to your actual measurements and history; that personalised answer is the one that counts.
Your dates
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The method
Naegele's rule dates from 1812 and is still the baseline: add 280 days (40 weeks) to the first day of the last menstrual period. The rule assumes a 28-day cycle with ovulation on day 14. Gestational age is counted from the same LMP date — which means the first two weeks of "pregnancy" happen before conception.
Source: ACOG — methods for estimating the due date · checked 2026-09-03
Frequently asked questions
How accurate is the Naegele due date?
Only about 4% of babies arrive on the due date. It is a reference point: the rule assumes a 28-day cycle and ovulation on day 14. Early ultrasound dating — which your clinician will do — is more precise.
How do I count weeks of pregnancy?
From the first day of the last period: that day is day 0 of week 0. The first trimester runs through the end of week 13, the second through week 26, and the third from week 27 to birth.
What if my cycle is not 28 days?
Naegele is 280 days from the last period start regardless. For longer or shorter cycles the real due date drifts with the ovulation day, which is exactly why clinicians date pregnancies by ultrasound rather than by calendar alone.
Count the days in one place
TempoLife tracks weight, sleep and habits through pregnancy — the trend your care team will ask about.