How this works
The standard pregnancy due date is calculated using Naegele's rule: take the first day of the last menstrual period (LMP) and add 280 days (40 weeks). The rule assumes a 28-day cycle with ovulation on day 14, so conception occurs about 14 days after the LMP, and pregnancy is roughly 38 weeks from conception. For longer cycles, ovulation is later, so the due date shifts later by the same number of days the cycle exceeds 28. This calculator does that adjustment automatically.
Naegele's rule gives the most-likely due date, but in practice only about 4% of babies are born exactly on it. Spontaneous labour typically occurs in a window from 38 to 42 weeks. Modern clinical practice uses an early-pregnancy ultrasound (8–13 weeks) to confirm the LMP-based estimate, since cycle irregularities and recall errors can shift the LMP date significantly. If your ultrasound dating differs from your LMP-based estimate by more than 7 days in the first trimester, doctors typically use the ultrasound date.
The formula
LMP is the first day of your last menstrual period (start of bleeding). cycle_length is your typical cycle length in days; 28 is the population average. The 280-day baseline assumes a 28-day cycle; the (cycle_length − 28) term adjusts for individual variation.
Example calculation
- LMP = January 1, cycle = 28 days. Due date = January 1 + 280 days = October 8.
- For a 32-day cycle: due date shifts to October 12 (4 days later, since cycle exceeds 28 by 4).
Frequently asked questions
How accurate is the due date?
It's a most-likely estimate, not a deadline. About 4% of babies arrive on the exact due date; about 50% arrive within ±5 days; about 90% within ±2 weeks. A baby born between 37 and 42 weeks is considered full term. Early ultrasound dating (before 13 weeks) is more accurate than LMP-based dating, especially for irregular cycles.
Why 280 days?
It comes from the average human gestation length: about 38 weeks (266 days) from conception, plus 14 days from LMP to ovulation in a typical 28-day cycle. 266 + 14 = 280 days = 40 weeks. The naming "first day of last period" rather than "conception" is a clinical convention because LMP is something women can usually recall, whereas exact conception date often isn't known.