A pregnancy is dated as 40 weeks, which is 280 days — about 9 months and a week. Yet almost nobody delivers exactly on the estimated due date. That gap between the arithmetic and the outcome confuses a lot of people, and it is entirely explainable. Here is the actual calculation clinicians use, why it is only a midpoint, and how to run the numbers yourself.
Naegele’s Rule: The Standard 280-Day Formula
The estimate clinicians call the EDD (estimated date of delivery) comes from Naegele’s rule, published in the early 1800s and still the default:
- Take the first day of the last menstrual period (LMP).
- Add 7 days.
- Subtract 3 months.
- Add 1 year.
Worked example: LMP = June 10, 2026. Add 7 days → June 17. Subtract 3 months → March 17, 2027. Add 1 year is already satisfied. EDD = March 17, 2027. That is 280 days from the LMP.
Why 280 Days Is a Convention, Not a Measurement
Naegele’s rule assumes a 28-day cycle with ovulation on day 14. Those assumptions fail for a large share of people:
| Assumption | Reality | Effect on EDD |
|---|---|---|
| 28-day cycle | Normal range 21-35 days | Shifts by up to ±7 days |
| Ovulation day 14 | Day 11-21 common | Shifts by up to ±7 days |
| Regular cycles | Irregular cycles common | Wide uncertainty |
This is why a first-trimester ultrasound is more accurate than the LMP date. Before 14 weeks, crown-rump length predicts gestational age to within about 5 days. After that, the margin widens, so clinicians keep the early scan date rather than revising later.
Gestational Age vs Fetal Age — Two Different Numbers
Here is a nuance that trips up almost everyone reading a scan report:
- Gestational age is counted from the LMP, so it includes roughly 2 weeks before conception.
- Fetal age (embryonic age) is counted from conception, so it is about 14 days less.
So “6 weeks pregnant” means the fetus has been developing for about 4 weeks. The two numbers describe the same pregnancy and differ by two weeks by definition.
How Often Does the Baby Actually Arrive on the Date?
Data from large obstetric cohorts consistently shows a spread rather than a spike:
- Roughly 4-5% of births occur on the exact EDD.
- About 80% occur within two weeks either side of it.
- Spontaneous labour most commonly begins between 38 and 42 weeks.
- Pregnancies beyond 42 weeks are considered post-term and are usually induced.
In other words, the EDD is the centre of a distribution, not a scheduled appointment.
How to Run the Calculation Yourself
If you want a working number without waiting for a clinic appointment, the arithmetic is straightforward once you have the LMP:
- Count forward 280 days.
- Alternatively, add 9 months and 7 days (equivalent for most months).
- Adjust by your average cycle length: subtract (cycle length − 28) days.
For a 32-day cycle, subtract 4 days from the standard estimate. For a 24-day cycle, add 4 days. Leap years add one day of slip if your window crosses February 29, which is the same leap-year bookkeeping that makes calculating an exact age in days trickier than it looks.
FAQ
Is 40 weeks the same as 9 months? No. 40 weeks is 280 days, roughly 9 months and 1 week.
Which is more accurate, LMP or ultrasound? An early ultrasound before 14 weeks is more accurate, typically within about 5 days.
Why does my app and my clinic give different dates? Different assumptions about cycle length and ovulation day, or one is using LMP and the other a dating scan.
What does “full term” mean? 39 to 40 weeks and 6 days is full term; 37-38 weeks is early term.
Date arithmetic with months of unequal length and leap years is exactly the kind of counting a dedicated tool handles cleanly — YourAgeCalculator runs the same day-level math for any pair of dates.
