How is my due date calculated?

The due date is calculated using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle. Only ~5% of babies arrive exactly on their due date; most arrive within 2 weeks before or after.

How accurate is the due date calculator?

Naegele's rule is accurate to within about 1-2 weeks for women with regular 28-day cycles. Only 5% of births occur exactly on the due date. Early ultrasound measurement (crown-rump length between 11-14 weeks) provides the most accurate dating.

What if my cycle isn't 28 days?

For longer or shorter cycles, you can adjust by adding or subtracting the difference from 28 days. For example, a 32-day cycle would add 4 days to the EDD. Early ultrasound is the most reliable method regardless of cycle length.

Due Date Calculator — Naegele's Rule

How the Due Date Is Calculated

This calculator uses Naegele's rule (the standard obstetrical method): add 280 days (40 weeks) to the first day of your last menstrual period (LMP). The rule assumes a 28-day menstrual cycle.

For non-28-day cycles, the calculator adjusts the EDD by the difference. For example, a 32-day cycle (4 days longer) adds 4 days to the due date.

Important: Naegele's rule is most accurate for women with regular cycles. Early ultrasound measurement between 11-14 weeks provides the most reliable dating.

Pregnancy Timeline

First Trimester (Weeks 1–12): Critical organ development. Most women discover pregnancy around week 4-6.
Second Trimester (Weeks 13–27): Baby becomes active; anatomy scan around week 20. Most women feel best during this period.
Third Trimester (Weeks 28–40): Baby grows rapidly, preparing for birth. Braxton-Hicks contractions may begin.

Track your baby's growth after birth

GrowChart tracks WHO growth percentiles from birth through age 5.

Start Tracking Free →

Due Date Calculator

How the Due Date Is Calculated

Pregnancy Timeline

Related Tools

Track your baby's growth after birth