When is my baby due?

The standard method is Naegele's rule: your estimated due date is 280 days (40 weeks) from the first day of your last menstrual period. It assumes a 28-day cycle with ovulation on day 14, so this tool adjusts for your real cycle length, a 32-day cycle pushes the date about four days later, a 26-day cycle two days earlier. If you know your conception or ovulation date instead, the due date is 266 days (38 weeks) from there, because pregnancy is dated from the last period but a baby is only actually conceived about two weeks later. Every method here resolves to the same internal anchor, so the due date, how far along you are, and your trimester always agree.

Almost certainly not on the exact day, and that is completely normal. Only about 1 in 20 babies is born on their estimated due date. A pregnancy is considered full term across a five-week window: 37 weeks 0 days to 42 weeks. Most people deliver in the two weeks on either side of the estimate, which is why clinicians treat the due date as the center of a range, not a deadline. Think of it as your best planning anchor for appointments, tests, and leave, not a delivery date.

The ultrasound, if it was early. ACOG's position (Committee Opinion 700) is that a first-trimester scan is the single most accurate way to date a pregnancy, more reliable than the last menstrual period, which depends on remembering the date and on a textbook cycle. Your provider redates the pregnancy to the scan when the two disagree by more than a set threshold: more than 5 days in the first trimester, more than 7 days between 14 and 16 weeks, and larger gaps later. Once a due date is set by an early accurate scan, it is not usually changed by later scans. This tool can estimate from either input, but a real scan read by your provider is the tiebreaker.

It is actually the most precise, because the exact age of the embryo is known rather than estimated. For a day-5 (blastocyst) transfer, the due date is 261 days from the transfer date; for a day-3 (cleavage-stage) embryo it is 263 days. There is no cycle-length guesswork and no uncertainty about when conception happened, so an IVF due date usually needs no ultrasound correction. Choose the IVF method above and enter your transfer date and embryo day.

It can, and this is the single biggest source of error in period-based dating. Naegele's rule assumes you ovulate on day 14; if you ovulate later, a real due date is later than the textbook calculation. Entering your true average cycle length here corrects for a consistently long or short cycle. But if your cycles are genuinely irregular, or you are not sure of your last period, an early dating ultrasound is the reliable answer, and it is exactly the situation ACOG recommends one for. Treat this estimate as a starting point to confirm at your first prenatal visit.

Yes. Conception happens about 266 days before the due date, which is roughly two weeks after the first day of your last period, around the time you ovulated. This tool shows an estimated conception window alongside your result. It is an estimate, not a paternity or legal determination: the fertile window spans several days and sperm can survive up to about five days, so conception could have occurred across a range of dates, not one.

Gestational age is how pregnancy is counted everywhere in medicine: from the first day of your last period. Fetal (or conceptional) age is the baby's true age from conception, which is about two weeks less. So at a 12-week ultrasound the fetus has actually been developing for about 10 weeks. Every week number on this page, and the ones your provider and your pregnancy apps use, is gestational age, so they all line up.

Completely. This calculator runs entirely in your browser. The dates you enter are never sent to a server, stored, or shared, and using it tells us nothing about you. Close the tab and everything is gone. It is an educational estimate, not medical advice or a diagnosis: your provider and an early scan confirm your dates.