Estimated due date: how it is set and how accurate it is

The usual rule adds 280 days to the first day of the last period

ACOG Committee Opinion 700, "Methods for Estimating the Due Date" (2017), states that by convention the estimated due date (EDD) is 280 days after the first day of the last menstrual period, or 40 weeks. The convention assumes a 28-day cycle with ovulation on day 14. It does not account for inaccurate recall of the last period, irregular cycles or variable timing of ovulation (ACOG, 2017). Because the count includes about two weeks before conception (ACOG, 2024), a date counted from conception is two weeks shorter: 266 days. The due date calculator on this site shows which method it used, and our method sets it out.

A scan or IVF can set the date instead

ACOG's patient guidance says the date is calculated from the first day of the last period, but that when the date of the last period is uncertain, an ultrasound examination in the first trimester may be used to estimate it. After IVF the date is set by the age of the embryo and the date of transfer (ACOG, "How Your Fetus Grows During Pregnancy", 2024). Committee Opinion 700 gives the arithmetic: for a day-5 embryo the due date is 261 days from the embryo replacement date, and for a day-3 embryo 263 days (ACOG, 2017). The NHS describes the same approach in England, where the sonographer at the dating scan estimates the due date from the baby's measurements (NHS, 2023). Due date based on ultrasound explains when the scan date replaces the last-period date.

About 1 in 20 babies arrive on the estimated date

ACOG says only about 1 in 20 women give birth on their due dates, and that the date is still useful: it determines gestational age throughout pregnancy so that fetal growth can be tracked, and it gives a timeline for tests during pregnancy (ACOG, 2024). A retrospective study of 18,708 women with spontaneous labor and a single baby without major anomalies, in a health district in Australia between 2007 and 2011, found that only 5 in 100 births occurred on the estimated date whichever dating method was used, and about 66 in 100 within 7 days of it (Khambalia et al., International Journal of Gynecology & Obstetrics, 2013). The NHS says pregnancy normally lasts from 37 weeks to 42 weeks from the first day of the last period (NHS, 2024). For that reason this site gives a range as well as a date.

When the estimated due date changes

ACOG advises that once the date is set from the last period, the first accurate ultrasound examination, or both, changes to it should be reserved for rare circumstances, discussed with the patient and documented (ACOG, 2017). The common reason is a first-trimester scan that differs from the last-period date by more than a set number of days. The NHS says a due date first calculated from the last period may be adjusted according to the ultrasound measurements (NHS, "Ultrasound scans in pregnancy", 2023).

UK and US differences

Both countries use the term estimated due date, or estimated date of delivery, and both start from the last period. England offers a dating scan to everyone: NICE guideline NG201 advises offering a scan between 11 weeks and 2 days and 14 weeks and 1 day to determine gestational age (NICE, 2021). ACOG's opinion describes how to use an ultrasound examination when one is done, and calls a pregnancy without one before 22 weeks and 0 days suboptimally dated (ACOG, 2017). The estimated gestational age guide explains the counting in more detail.

This guide is information, not a diagnosis or a recommendation of treatment, and it is no substitute for your own clinician. See the medical disclaimer.