Pregnancy Due Date & Gestational Age Calculator

Naegele's rule dates a pregnancy from the first day of the last menstrual period and adds 280 days, assuming ovulation on day 14 of a 28-day cycle. Longer or shorter cycles shift ovulation, so the cycle-length adjustment corrects the estimate. An IVF transfer has a known conception date, so no adjustment applies — a 5-day blastocyst is already 19 days into gestational age at transfer. Worked example: a last menstrual period starting on 15 January 2026, with a regular 28-day cycle, gives a due date 280 days later, on 22 October 2026. A longer or shorter cycle moves the date by the number of days it differs from 28. An early dating ultrasound is more accurate than the last menstrual period, so your doctor or midwife may adjust the date. Only about 5% of babies are born on the due date, and a healthy birth any time from 37 to 42 weeks is normal.