How due dates are estimated
Pregnancy is counted from the first day of your last menstrual period (LMP), not from conception, because most people know when their period started and few know exactly when they ovulated. A full-term pregnancy is 40 weeks, or 280 days, from the LMP. That is Naegele's rule: take the first day of the last period, add one year, subtract three months and add seven days. An LMP of January 1, 2026 gives a due date of October 8, 2026. The rule assumes a 28-day cycle with ovulation on day 14; if your cycles are longer or shorter, this calculator shifts the date by the difference, so a 32-day cycle pushes the example to October 12.
If you know the conception date, whether from ovulation tracking or a single encounter, the due date is 266 days later, which is the same 280-day clock started 14 days in. IVF pregnancies are the most precisely dated of all: a day-5 blastocyst transfer is treated as conception five days earlier, so the due date is transfer date plus 261 days; a day-3 transfer is plus 263 days.
Dating by ultrasound
A first-trimester ultrasound measures the embryo's crown-rump length and converts it to a gestational age, accurate to within about 5-7 days. The American College of Obstetricians and Gynecologists recommends using the ultrasound date instead of the LMP date whenever the two disagree by more than 5 days before 9 weeks, 7 days between 9 and 16 weeks, and by larger margins later on. If your provider gave you a gestational age at a scan, choose the ultrasound option, enter the scan date and the weeks and days you were told, and the calculator will work backward to the equivalent LMP and forward to the due date. Once a due date has been set by an early ultrasound it is normally not changed by later scans, even if the baby measures big or small.
How far along you are, and what the milestones mean
Gestational age is reported in completed weeks and days, so "10 weeks, 3 days" means you are in your 11th week. Trimesters are defined by ACOG as weeks 0 through 13 and 6 days, 14 through 27 and 6 days, and 28 onward. The milestone table lists the dates for 12 weeks (end of the first trimester, when miscarriage risk drops sharply and first-trimester screening happens), 20 weeks (the anatomy scan), 28 weeks (start of the third trimester, glucose screening and Rh immune globulin if you are Rh-negative), 37 weeks (early term), 40 weeks (your due date) and 42 weeks (post-term, when induction is usually discussed).
Only about 4-5% of babies are born on their due date. Roughly 70% arrive within 10 days of it, and first babies tend to come a few days later than average. Treat the date as the middle of a window rather than an appointment. Your provider will confirm and, if necessary, adjust the date at your first prenatal visit; nothing on this page replaces that conversation.