DueDateCalculatorduedate-calculator.com

Due Date Accuracy: The Research

Your due date is an estimate — and a useful one. But understanding just how accurate it is helps set realistic expectations for birth planning.

Ashok Kumar Poudel
Written by
Ashok Kumar Poudel
Health & Wellness Writer
Dr. Bina Basnet
Medically reviewed by
MBBS, MD — Gynecologist & Obstetrician
Last reviewed:
Feb 2026
~50%
Within 7 days
Half of all births occur within one week of the EDD
~70%
Within 10 days
Most births occur within 10 days of EDD
~85%
Within 14 days
Vast majority within 2 weeks
4–5%
Exact date
Only 1 in 20 babies born on EDD
~5%
2+ weeks early
Spontaneous preterm at 37–38 wks
~5%
2+ weeks late
Post-term (42+ weeks)

The Science Behind Due Date Variability

A key study published in Human Reproduction (Jukic et al., 2013) followed 125 women with precisely known conception dates. The researchers found that gestation length ranged from 247 to 284 days — a 37-day spread — even among healthy, normal pregnancies. The standard deviation was 9.6 days, meaning two-thirds of births fall within about 10 days of the mean.

Naegele’s Rule, developed in 1812, is based on the mean of 280 days. While accurate as a population average, it cannot predict individual variation. This is why only about 5% of births occur on the exact EDD.

Factors That Affect Due Date Accuracy

Irregular menstrual cycles
Can shift EDD by days to weeks; cycle length adjustment improves accuracy
Uncertain LMP date
Common cause of dating errors; first-trimester ultrasound corrects this
Late first ultrasound (>14 weeks)
Second-trimester biometry is less precise than first-trimester CRL
Multiple pregnancies (twins)
Twin pregnancies typically deliver 3–4 weeks before singleton EDD
First pregnancy (primigravida)
Slight tendency to deliver after EDD vs subsequent pregnancies
Individual biological variation
Genetic and physiological factors affect gestation length independently of any formula

How to Get the Most Accurate Due Date

  1. Start with LMP — use a confirmed, accurate LMP date
  2. Adjust for cycle length — especially if not a 28-day cycle
  3. Get an 8–14 week ultrasound — CRL measurement is accurate to ±5 days
  4. For IVF — use the transfer date formula for the most precise EDD
  5. Don't change after 14 weeks — later ultrasounds are less precise for dating

Frequently Asked Questions

Common questions answered by our medical team

Human gestation length naturally varies by weeks between individuals. Additionally, LMP-based dates assume ovulation at day 14, which varies. Even with ultrasound dating, biological variation in fetal growth means birth timing has a wide normal range.
Calculate Your Due Date →