Due Date Calculator

How Due Date Calculation Works

The most common method of calculating a due date is Naegele's Rule, which adds 280 days (40 weeks) to the first day of the last menstrual period. This assumes a regular 28-day cycle and ovulation on day 14. When calculating from a known conception or ovulation date, 266 days (38 weeks) are added, reflecting the actual length of fetal development. For more detailed tracking of your pregnancy journey — including trimester boundaries and milestone appointments — check out our Pregnancy Calculator.

IVF due date calculations differ slightly because the exact age of the embryo is known at the time of transfer. A 5-day blastocyst transfer uses 261 days from the transfer date (equivalent to 40 weeks minus 19 days), while a 3-day embryo transfer uses 263 days. These figures are standard in reproductive medicine and account for the known developmental stage of the embryo. If you want to understand the conception timeline further, our Conception Calculator can estimate when fertilization likely occurred based on either your due date or LMP.

It is worth noting that due dates are estimates, not guarantees. Doctors often revise the due date after a first-trimester ultrasound, especially if cycle length is irregular or LMP is uncertain. A normal, healthy pregnancy can end anywhere from 37 to 42 weeks. Babies born between 39 and 40 weeks are considered "full term," while those born at 37–38 weeks are "early term." To track your weight gain throughout pregnancy using IOM guidelines, visit our Pregnancy Weight Gain Calculator.