Due Date Calculator
Estimated due date from your last menstrual period (LMP) using Naegele's rule.
Enter the first day of your last period to see the estimated due date.
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About this tool
Naegele's rule is the standard clinical method to estimate delivery date: add 7 days to the first day of the last menstrual period, then add 9 months. That gives a 40-week pregnancy from a 28-day cycle. Actual delivery is any time in a ~5-week window around the estimate — only about 4% of babies arrive exactly on the due date.
Formula & notes
Due date = First day of LMP + 280 days (40 weeks). The formula assumes a 28-day cycle with ovulation on day 14; if your cycle is longer or shorter, adjust by (cycle length − 28) days. The calculator shows the estimated due date, current gestational age in weeks + days, and current trimester (T1: weeks 1-13, T2: 14-27, T3: 28+). Not a substitute for an ultrasound dating scan; see your provider for a clinical estimate.
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