Estimate your pregnancy due date, conception date and current gestational week from the first day of your last menstrual period and your cycle length.
Enter the first day of your last menstrual period and your average cycle length. The calculator estimates your due date (Naegele's rule), conception date, current week and trimester.
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A due date calculator estimates when your baby will arrive using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period, adjusted for your cycle length. Only about 5% of babies arrive on the exact date — it's an estimate.
Due date = LMP + 280 days, plus an adjustment of (cycle length − 28) days for cycles that aren't 28 days. Conception is assumed ~14 days after LMP. Trimesters: 1st weeks 1–12, 2nd 13–26, 3rd 27–40.
It's a best estimate. An early dating ultrasound (before 12 weeks) is more precise. Most babies arrive within two weeks of the estimate, on either side.
Naegele's rule assumes regular cycles. With irregular cycles the estimate is less reliable — an ultrasound dating scan is the better guide.