How due dates are calculated
A due date is a calendar estimate of 40 weeks — 280 days — from the first day of the last menstrual period (LMP). That number is older than it looks: it comes from Naegele’s rule, published by the German obstetrician Franz Karl Naegele in the early 19th century, built on an idea from the 1700s that pregnancy lasts ten lunar months. Add one year to the LMP, subtract three months, add seven days — and you get the same date this calculator produces.
Three starting points are supported, because you rarely know all three:
- Last period — LMP + 280 days. This assumes ovulation on day 14 of a 28-day cycle, so the calculator shifts the date by one day for every day your average cycle differs from 28. A 33-day cycle pushes the due date five days later.
- Conception date — conception + 266 days. Same arithmetic, minus the two weeks between the period and ovulation. Useful if you tracked ovulation precisely; our conception calculator works this logic in reverse.
- IVF transfer — the only method with no guesswork. A day-5 blastocyst is due 261 days after transfer; a day-3 embryo, 263 days.
Why ultrasound dating overrides the calendar
The LMP method has two soft spots: it trusts your memory of the period date, and it assumes a textbook ovulation day. Real ovulation moves — stress, travel and normal cycle variation shift it by days. A first-trimester ultrasound sidesteps both problems by measuring the embryo’s crown-rump length, which grows so predictably in early pregnancy that the scan can date it within about five days. That is why providers redate a pregnancy when the early scan disagrees with the period-based date by more than roughly a week — and why the scan date, once set, doesn’t change again even if later scans drift.
The due date is a window, not a day
Only about 4–5% of babies arrive on their estimated due date. Around 90% arrive between 37 and 42 weeks — the window the calculator shows — and in US birth data the single most common week of delivery is week 39, not week 40. Part of that is biology; part is that inductions and scheduled cesareans cluster at 39 weeks. Obstetrics formalizes the window with four named zones:
| Term | Weeks | What it means |
|---|---|---|
| Early term | 37w0 – 38w6 | Mature for most babies, but lungs and feeding often benefit from more time |
| Full term | 39w0 – 40w6 | The sweet spot — best average outcomes; elective delivery is scheduled here |
| Late term | 41w0 – 41w6 | Still normal; monitoring typically increases |
| Post-term | ≥ 42w0 | Uncommon (~under 1% today); induction is usually offered before this point |
A practical way to hold the number: plan around the two weeks either side of the due date, and treat anything from 39 weeks onward as “any day now.” First pregnancies run slightly longer on average than subsequent ones, and a family history of longer pregnancies nudges the odds too — none of which any calculator can see.
What to do with the date
The due date anchors everything else in prenatal care: when screening tests are valid, when the anatomy scan is scheduled, when a baby would be considered preterm, and when an induction conversation starts. Once you have it, our pregnancy calculator turns the same date into a week-by-week timeline of where you are now and what milestone comes next, and the pregnancy weight gain calculator shows the recommended gain curve for your build across those 40 weeks.
One caveat worth repeating: everything on this page is an estimate for orientation, not medical advice. Your provider’s dating — especially after a first-trimester ultrasound — is the date that counts.