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Pregnancy Due Date Calculator
Estimate gestational age, your current trimester and an approximate due date from the first day of your last period.
This tool provides a general estimate only and is not a substitute for professional medical advice; talk to a doctor about decisions affecting your personal health. Always confirm your exact due date with an ultrasound and your care provider.
How is the due date calculated?
This calculator uses Naegele's rule: it adds 280 days (40 weeks) to the first day of your last menstrual period (LMP), then adjusts for cycles that run longer or shorter than the standard 28 days. Gestational age is simply how many days have passed since your LMP, expressed in completed weeks and days; pregnancy is conventionally divided into three trimesters, with the second beginning around week 13 and the third around week 28.
Naegele's rule is a well-known starting estimate, not a guarantee — only about 5% of babies arrive on their exact due date, and most are born within a couple of weeks either side of it. An early ultrasound, which measures the size of the developing embryo or fetus directly, is generally considered more accurate than LMP-based dating alone, especially if your cycles are irregular.
Frequently asked questions
What is Naegele's rule?
A standard method for estimating a due date: add 280 days (40 weeks) to the first day of your last menstrual period, adjusted for your average cycle length.
How accurate is a due date calculated this way?
It's a useful estimate, but only a small share of babies are born on their exact due date — most arrive within about two weeks of it, in either direction.
What if my cycles are irregular?
Naegele's rule assumes a fairly regular cycle. If yours varies a lot, an early ultrasound dating scan is usually more reliable than LMP-based calculations.
How are the trimesters defined?
This tool treats weeks 1–13 as the first trimester, weeks 14–27 as the second, and week 28 onward as the third, which is a common convention.