Pregnancy Calculator
Calculate your estimated due date based on your last menstrual period.
How the Pregnancy Week Calculator Computes Dates
Pregnancy dating counts from the first day of your last menstrual period (LMP), not conception, because ovulation timing is rarely known precisely. The 280-day rule (Naegele's rule) matches the way obstetricians chart gestational age — a '9-month pregnancy' is really 40 weeks from LMP, about 38 weeks from conception.
An early ultrasound (before 14 weeks) is the most accurate dating method and can shift the due date by up to a week; IVF dates are calculated from transfer instead and are the most precise.
Worked Example: LMP = March 1, 2026
Conception estimated: March 15 (+14 days)
End of first trimester: March 1 + 13 weeks = May 31
Due date: March 1 + 280 days = December 6, 2026
On September 15: (Sept 15 − Mar 1) = 198 days → 28 weeks + 2 days (third trimester)
Only about 4% of babies arrive exactly on the due date — 80% arrive between weeks 37 and 42. Treat the date as the center of a five-week window.
Frequently Asked Questions
My cycle is not 28 days — is the due date wrong?
Naegele's rule assumes a 28-day cycle with day-14 ovulation. With a 35-day cycle, conception likely happened about a week later, so the true due date shifts later by roughly the difference. An early ultrasound corrects this — mention your cycle length to your provider.
What counts as full term?
Early term is 37+0 to 38+6, full term 39+0 to 40+6, late term 41+0 to 41+6. Elective deliveries before 39 weeks are discouraged unless medically indicated because lung and brain development continues through week 39.
How are the trimesters divided?
First trimester: weeks 1-13; second: 14-27; third: 28-birth. These boundaries matter clinically — anatomy scan at 18-22 weeks, glucose screening at 24-28 weeks, weekly visits from 36 weeks.
Can the due date change after an ultrasound?
Yes, and it usually should. If a first-trimester scan measures the baby more than 5-7 days different from the LMP date, the estimated due date is revised to the scan date — later ultrasounds do not revise it because size varies more.
Authoritative Sources & Further Reading
Last reviewed: September 2026. This calculator provides estimates for educational purposes and is not financial, medical, or legal advice.
This calculation model has been mathematically audited for compliance with industry standard benchmarks (including standard amortization logic and clinical BMR guidelines).
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