Due Date Calculator

How Pregnancy Due Dates Are Calculated

A due date (estimated date of delivery, or EDD) is calculated by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). This is known as Naegele's rule, developed in the early 19th century and still the standard method used by obstetricians worldwide. The 40-week calculation assumes a 28-day cycle with ovulation occurring on day 14.

Using the Calculator

Enter the first day of your last menstrual period and your average cycle length. The calculator shows your estimated due date, current gestational age in weeks and days, trimester, and key milestone dates throughout pregnancy. If your cycle is longer or shorter than 28 days, the calculator adjusts the due date accordingly.

Trimester Milestones

First trimester (weeks 1-12): Major organ formation occurs. First prenatal visit typically at 8-10 weeks. First ultrasound confirms viability and may adjust the due date. Second trimester (weeks 13-27): Anatomy scan at 18-22 weeks. Fetal movement felt around 16-25 weeks. Glucose screening at 24-28 weeks. Third trimester (weeks 28-40): Growth monitoring, Group B strep test at 36 weeks, weekly visits from 36 weeks onward.

Due Date Accuracy

Only about 5% of babies are born on their exact due date. The due date represents the midpoint of a normal delivery range — full term is defined as 39 weeks 0 days through 40 weeks 6 days by the American College of Obstetricians and Gynecologists. Deliveries between 37 and 42 weeks are considered within the normal range. First pregnancies tend to run slightly longer on average.

Ultrasound Dating vs LMP Dating

A first-trimester ultrasound measuring crown-rump length is the most accurate method for determining gestational age — accurate to within ±5-7 days. If the ultrasound date differs from the LMP-calculated date by more than 7 days, the ultrasound date is generally preferred. Later ultrasounds are less precise for dating (±2-3 weeks in the third trimester).

Cycle Length Adjustments

Naegele's rule assumes a 28-day cycle. For longer cycles, ovulation occurs later, pushing the due date forward. A woman with a 35-day cycle likely ovulated around day 21 instead of day 14 — adding 7 days to the standard due date. The calculator makes this adjustment automatically when you enter your average cycle length.

Authoritative Sources

This calculator uses formulas and reference data drawn from the following sources:

- CDC — Physical Activity - NIH — National Institute of Diabetes - NIH — National Heart, Lung, and Blood Institute

This due date calculator uses standard health formulas to compute results. Enter your values and the formula is applied automatically — all math is handled for you. The calculation follows industry-standard methodology.

• https://www.nih.gov
• https://www.who.int
• https://www.cdc.gov