The standard method is Naegele’s rule: take the first day of your last menstrual period and add 280 days, or equivalently add one year, subtract three months and add seven days, to get the estimated due date at 40 weeks. This assumes a regular 28-day cycle with ovulation around day 14, so if your cycle is longer or shorter the calculator shifts the date by the difference. AnyTool does this live and also shows your current gestational age in weeks and days.

Gestational age is dated from the first day of your last menstrual period (or the effective LMP derived from a conception date, IVF transfer, due date or ultrasound). The calculator counts the whole days to your chosen reference date, divides by seven for the completed weeks and keeps the remainder as the extra days, then maps it to a trimester: first is weeks 1–13, second is 14–27 and third is 28–40 and beyond. Because it is dated from the LMP, gestational age is about two weeks more than the time since conception.

IVF dating is precise because the embryo’s age is known: AnyTool sets the due date to the transfer date plus 261 days for a day-5 blastocyst or plus 263 days for a day-3 cleavage embryo. For the ultrasound method you enter the gestational age the scan reported (weeks and days) on the scan date, and the calculator back-dates the effective LMP from it; a first-trimester ultrasound is the most accurate way to date a pregnancy. You can also reverse-engineer everything from a due date you were already given.

Only about 4–5% of babies are born exactly on their due date, and a healthy term birth can happen any time from 37 to 42 weeks, so treat the date as a window rather than a deadline. The LMP method assumes a regular 28-day cycle, which can be off by several days for irregular cycles, and your provider’s first-trimester ultrasound dating is the most accurate reference. AnyTool Pregnancy Calculator is informational only and not medical advice; every calculation runs locally in your browser and the page works offline after first load.