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.
Common Mistakes & Pro Tips
Avoid these mistakes
Counting from conception instead of LMP
Clinical pregnancy age counts from the last menstrual period (LMP), so '9 weeks pregnant' is about 7 weeks from conception. Due-date calculators (Naegele's rule: LMP + 280 days) follow the medical standard; conception-based dates run 2 weeks behind clinic charts.
Trusting the due date as a deadline
Only about 4–5% of babies arrive exactly on the estimated date; 80% arrive within 10 days of term. Treat the date as the center of a window and plan commitments for weeks 37–42 — the calculator shows the week-based window for exactly this reason.
Converting weeks to months by dividing by 4
Obstetric months vary (40 weeks = 9.2 calendar months). Communicate in gestational weeks — every pregnancy chart and clinician uses weeks, never month arithmetic.
Pro tips
Trimester boundaries in weeks
First trimester: weeks 1–13; second: 14–27; third: 28 to birth. Book the anomaly scan at 18–21 weeks and the glucose test at 24–28 — the week counter keeps every appointment inside its official window.
Adjust for cycle length
Naegele assumes a 28-day cycle with ovulation at day 14. If cycles run 35 days, ovulation was near day 21 and LMP-based dating overestimates by a week — the 7–13 week ultrasound is the tiebreaker.
Track fundal height alongside weeks
From about 20 weeks, fundal height in cm should roughly equal gestational weeks (plus or minus 2). It is the low-tech cross-check that week count and growth agree — bring both numbers to prenatal visits.
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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