Due Date Calculator

From your last period it uses Naegele's rule: 280 days (40 weeks) added to the first day of your last menstrual period, adjusted by your average cycle length. From a conception date it adds 266 days. For IVF it counts 261 days from a day-5 transfer or 263 days from a day-3 transfer, and from an ultrasound it projects forward from the gestational age measured at the scan.

It is a genuine estimate, not a promise: only around 4–5% of babies are born on their exact due date, and most arrive within two weeks either side of it. A first-trimester ultrasound is the most accurate way to date a pregnancy, which is why your healthcare provider may adjust the calculated date after an early scan.

Enter your date and the calculator shows your gestational age today in weeks and days, plus your current trimester. Note that pregnancy is counted from the first day of the last period, so "gestational age" runs about two weeks ahead of the time since conception — a common source of confusion.

The week-by-week timeline in this calculator shows an approximate length and weight for every week — from a poppy seed at week 4 (about 1 mm) to a watermelon-sized baby of roughly 3.5 kg around week 40 — along with what is developing at each stage. The figures are averages from standard fetal growth charts: every baby grows at its own pace, so your ultrasound measurements are the reference that counts.

The 280-day rule assumes a 28-day cycle with ovulation around day 14. If your cycles are consistently longer or shorter, set your average cycle length and the estimate shifts accordingly. With very irregular cycles the last-period method becomes unreliable, and a dating ultrasound (or the conception date, if known) gives a better estimate.

Yes. The whole calculation runs locally in your browser with JavaScript — the dates you enter never leave your device, nothing is stored, and there is no sign-up. It is a free informational tool, not medical advice: always confirm dates and decisions with your healthcare provider.