Pregnancy Due Date Calculator
Estimate your due date and how far along you are, from your last period or a known conception date.
Naegele's rule
From conception: + 266 days (38 weeks)
Pregnancy is counted from the first day of the last period — about two weeks before conception — so "40 weeks" reflects roughly 38 weeks of actual gestation. If your cycle isn't 28 days, the estimate shifts by the difference from 28.
Conception date March 1: due date = March 1 + 266 days = November 23. Inferred last-period date = March 1 − 14 days = February 15.
Compare this to the LMP method: if that same person's actual last period was February 15 but their cycle ran 32 days instead of 28, the LMP-based due date would shift 4 days later than the conception-based one (280 + 4 = 284 days from February 15), landing on November 27 instead of November 23. The two methods only agree exactly when the cycle is a standard 28 days — this is why a known conception date (from fertility tracking or IVF) is generally treated as more reliable than a last-period estimate when cycles are irregular.
What the estimate means
A due date is a helpful midpoint, not a deadline: only about 1 in 20 babies arrive on the exact day, and a "full-term" birth is anywhere from 37 to 42 weeks. An early dating ultrasound (before roughly 13 weeks) is more accurate than period-based dating, particularly with irregular cycles — your provider's date takes precedence over any calculator. Track healthy weight gain over the pregnancy with the pregnancy weight gain calculator.
Full-term ranges (ACOG classification)
| Category | Gestational age |
|---|---|
| Preterm | Before 37 weeks 0 days |
| Early term | 37 weeks 0 days – 38 weeks 6 days |
| Full term | 39 weeks 0 days – 40 weeks 6 days |
| Late term | 41 weeks 0 days – 41 weeks 6 days |
| Postterm | 42 weeks 0 days and beyond |
Per the American College of Obstetricians and Gynecologists (ACOG), any birth from early term through late term — roughly a five-week span — is considered medically typical. The single "due date" this calculator returns sits near the middle of the full-term window, not at the edge of a deadline.
Common due-date mistakes
- Reading "40 weeks" as 40 weeks of actual gestation. It's counted from the last period, about two weeks before conception, so true fetal development is closer to 38 weeks by the due date.
- Treating the due date as a deadline rather than a midpoint of a range. Only about 5% of babies are born on the exact calculated date; the full-term window spans multiple weeks (see the table above).
- Not adjusting for cycle length when it isn't 28 days. As the conception-date example above shows, a longer or shorter average cycle shifts ovulation — and therefore the due date — by the same number of days.
Frequently asked questions
How is a due date calculated?
Naegele's rule: last period + 280 days (or conception + 266 days).
How accurate is it?
An estimate — ~5% arrive on the date; most within two weeks. Ultrasound dating is more precise.
What are the trimesters?
First 1–13, second 14–27, third 28–40 weeks, counted from the last period.
Will my due date change after an ultrasound?
It might — a first-trimester ultrasound measuring the embryo directly is considered more reliable than counting from the last period, especially with irregular cycles.
What counts as "full-term" versus my due date?
Full-term is a range, not one day: ACOG defines it as 39-40 weeks 6 days, with early term (37-38 weeks) and late term (41 weeks) also considered medically typical.
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Medical disclaimer: This provides an estimate only and is not medical advice or a substitute for prenatal care. Your healthcare provider's due date, based on examination and ultrasound, is authoritative. Formula: Naegele's rule. Last reviewed: September 2026.