How your due date is calculated
The standard estimate — the one printed on scan reports — is Naegele's rule: 280 days (40 weeks) from the first day of your last menstrual period. That convention counts roughly two weeks before conception, because periods are easier to date than ovulation. If your cycles run longer or shorter than 28 days, ovulation shifts too, so this calculator moves your due date one day for every day your cycle differs from 28.
Four ways to date a pregnancy
Last period (LMP) is the everyday method and works well with regular cycles. Conception date adds 266 days and suits people who tracked ovulation. IVF transfers are the most precise of all — the embryo's age is known to the day, so a day-5 transfer adds 261 days and a day-3 transfer adds 263.
How accurate is it?
An ultrasound before 14 weeks dates a pregnancy to within about 5–7 days and, when it disagrees meaningfully with period-based dates, clinicians usually adopt the scan date. Even then, a due date is a midpoint, not a deadline: roughly 4% of babies arrive on the estimated day, and the large majority arrive between 37 and 42 weeks.
Trimesters and full term
The second trimester begins at 14 weeks 0 days and the third at 28 weeks 0 days. Early term runs 37w0d–38w6d, full term 39w0d–40w6d, late term 41w0d–41w6d, and post-term from 42 weeks — the vocabulary your provider will use when discussing timing near the end.
What to do with the date
Book a first (dating) scan for weeks 8–14, use the trimester dates to plan screening windows, and treat the final month as a range. Save the calculation to your ListCalc history so the milestone dates travel with you.
ListCalc health tools are educational estimates, not medical advice. Dates and ranges here are population averages — your midwife, OB-GYN or doctor confirms what applies to you. See our disclaimer.