Pregnancy and due date calculator

The estimate uses Naegele's rule, the classic prenatal method. From the first day of the last menstrual period (LMP), add 280 days — 40 weeks, the average length of a pregnancy counted from the LMP.

Due date = LMP + 280 days

The traditional form is equivalent: add 7 days to the LMP, subtract 3 months, add 1 year.

Gestational age and trimesters

Gestational age is the time from the LMP to today, in weeks and days (e.g., "12 weeks and 3 days"). It splits pregnancy into trimesters:

• 1st trimester: up to 13 weeks
• 2nd trimester: 14 to 27 weeks
• 3rd trimester: 28 weeks to birth

Why it's counted from the last period

Almost no one knows the exact date of conception, but most people remember their last period. Since ovulation happens ~2 weeks after the LMP, gestational age is always about 2 weeks "ahead" of the actual time since fertilization — this is expected and standardized worldwide.

Important: it is an estimate. A first-trimester ultrasound is the most accurate method and may adjust the date. Always use it alongside your doctor.