Pregnancy Calculator
Due date from the first day of your last menstrual period (standard 40-week assumption).
Last menstrual period
Estimated due date:
Gestational age uses medical convention (LMP). Confirm dating with your clinician.
The 280-day (40-week) rule
This calculator adds 280 days (40 weeks) to the first day of your last menstrual period — the standard clinical convention (also known as Naegele's rule) for estimating a due date, based on the assumption of a 28-day cycle with ovulation around day 14.
Frequently asked questions
What if my cycle isn't exactly 28 days? The 280-day rule assumes a 28-day cycle — if yours is longer or shorter, your actual ovulation (and therefore conception) likely happened on a different day than this rule assumes, which is why an early ultrasound is often used to refine dating.
Is my due date a guaranteed delivery date? No — only a small percentage of babies are born exactly on their estimated due date. It represents the middle of a normal range; full-term delivery is generally considered anywhere from about 3 weeks before to 2 weeks after.
Why is gestational age counted from LMP instead of conception? It's a long-standing clinical convention that predates reliable ovulation tracking — counting from a known, reportable date (last period) was more practical than trying to pinpoint the exact conception date.