Your results (calculated as of today)
The LMP is estimated by subtracting 280 days from your due date (the reverse of Naegele's rule). For general information only — not medical advice.
How the reverse due-date calculation works
Your due date is normally calculated from your last menstrual period using Naegele's rule: due date = LMP + 280 days. This tool simply reverses that equation. When you enter your due date, it subtracts 280 days to estimate your last menstrual period, and then counts forward again to show how far along you are today.
For example, a due date of May 8, 2027 points back to a last menstrual period around August 1, 2026. If that date matches your records, the calculator will report your current gestational age, which trimester you are in, and how many days remain until your due date.
The result is an estimate. Due dates themselves are estimates — they can be revised by an early ultrasound, which measures the embryo more precisely than a calendar can. If your provider has given you a due date that was adjusted by ultrasound, use that date here for the most accurate reverse calculation.
It is worth remembering that gestational age is counted from your last period, not from conception, so the first two weeks of "pregnancy" happen before you are technically pregnant. A full-term pregnancy spans 37 to 42 weeks, and only a small fraction of babies arrive exactly on their due date — so a few days of difference either way is completely normal.
Once you know your week, you can check the week-by-week guide to see what is happening with your baby and your body right now. Or use the main calculator to enter your LMP or week directly.
Whether you are in the first, second, or third trimester, knowing your gestational age is the starting point for every checkup and milestone along the way.