Pregnancy Due Date Calculator

Enter the first day of your last menstrual period and the calculator returns an estimated due date using Naegele's rule: add one year, subtract three months, add seven days. The same math defines your trimester boundaries (first ends at 13 weeks, second ends at 27 weeks, third runs to delivery).

Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is shorter or longer than 28 days, the calculator adjusts: shift the due date by the difference between your average cycle length and 28 days.

Due dates are estimates, not appointments. Only about 4% of babies are born exactly on the calculated date; most arrive within a two-week window around it. An ultrasound dating scan in the first trimester is more accurate than LMP-based math, especially for irregular cycles.

The rule's history: developed by Franz Karl Naegele in the early 1800s, it's served as the standard estimation method for nearly two centuries because the math is simple enough to do mentally. The 280-day total (40 weeks) breaks down as 38 weeks of actual fetal development plus the ~2 weeks of pre-conception cycle time the count starts from. That's why 'gestational age' and 'fetal age' differ by about two weeks — when an OB says you're 6 weeks pregnant, the embryo is 4 weeks old.

Worked example: LMP was January 15. Add 1 year: January 15, next year. Subtract 3 months: October 15. Add 7 days: October 22. Estimated due date October 22. If your cycles run 32 days (4 days longer than 28), shift the due date 4 days later: October 26. If your cycles run 24 days, shift 4 days earlier: October 18. The cycle adjustment is approximate — ovulation timing varies even within a regular cycle, and progesterone-confirmed ovulation timing (used in fertility tracking) is more accurate than counting from LMP for cycles that aren't textbook.

When LMP-based math fails: irregular cycles, recent hormonal birth control discontinuation, breastfeeding-induced amenorrhea, or any situation where you're not sure when (or whether) you ovulated. In those cases, an early ultrasound — typically performed between 8 and 13 weeks — measures the embryo's crown-rump length and provides a dating estimate accurate to within 5-7 days. After 13 weeks, ultrasound dating becomes progressively less reliable as natural fetal size variation widens. If the LMP-based date and the ultrasound-based date differ by more than a week in the first trimester, clinicians typically use the ultrasound date as the working estimate.