A due date feels like it should be a precise, guaranteed moment. It isn’t. It’s a statistical estimate, and understanding how it’s actually calculated helps set realistic expectations for the weeks around it. Here’s the method, with a real worked example.
Naegele’s Rule: The Standard Due Date Formula
The most common method, called Naegele’s Rule, calculates a due date as:
Due Date = First Day of Last Menstrual Period (LMP) + 280 days
That’s equivalent to 40 weeks from the first day of your last period, not from the actual date of conception, which typically happens about two weeks later.
Real Example: LMP of January 10
Let’s calculate.
- Last menstrual period (LMP): January 10, 2026
- Add 280 days (40 weeks)
Estimated due date: October 17, 2026
Trimester Breakdown for This Example
Using the same LMP date, here’s how the trimesters fall:
- First trimester: January 10 – April 11 (weeks 1–13)
- Second trimester: April 11 – July 18 (weeks 14–27)
- Third trimester: July 18 – October 17 (weeks 28–40)
Why Due Dates Are Estimates, Not Guarantees
Only a small percentage of babies are actually born on their exact calculated due date. The American College of Obstetricians and Gynecologists notes that a full-term pregnancy is generally considered anywhere from 37 to 42 weeks, meaning the due date represents the statistical middle of a fairly wide normal range, not a fixed deadline.
LMP Dating vs Ultrasound Dating
Naegele’s Rule assumes a textbook-regular 28-day cycle with ovulation on day 14. Many people don’t have perfectly regular cycles, which is exactly why early ultrasound dating (typically done in the first trimester) is often considered more accurate than LMP-based calculation alone. Ultrasound measures the fetus directly, rather than relying on menstrual cycle assumptions, and doctors will often adjust the calculated due date if the ultrasound measurement differs meaningfully from the LMP-based estimate. The Mayo Clinic’s guide to due date calculation explains how these two methods can be reconciled when they disagree.
Calculating From Conception Date Instead
If you know your actual conception date (less common, but possible with fertility tracking or IVF), the calculation shifts slightly:
Due Date = Conception Date + 266 days
This method skips the LMP-to-ovulation assumption entirely, since it starts from the actual fertilization date rather than estimating backward from the last period.
Factors That Can Shift the Estimate
- Irregular menstrual cycles — a cycle significantly longer or shorter than 28 days shifts the actual ovulation date, making pure LMP-based calculation less reliable
- First pregnancy vs subsequent pregnancies — first pregnancies statistically run slightly longer on average than subsequent ones
- Multiples — twin and multiple pregnancies typically deliver earlier than the standard single-baby due date calculation would suggest
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Frequently Asked Questions
How accurate is a due date calculated from LMP?
It’s a reasonable estimate but not precise, since it assumes a standard 28-day cycle with ovulation on day 14, which doesn’t apply to everyone. Early ultrasound dating is generally considered more accurate, especially for those with irregular cycles.
What is considered full term?
Full term is generally defined as 39 to 40 weeks 6 days, while the broader range of 37 to 42 weeks is considered within normal range for delivery timing, according to standard obstetric guidelines.
Can my due date change during pregnancy?
Yes, doctors sometimes adjust the calculated due date based on first-trimester ultrasound measurements if they differ meaningfully from the LMP-based estimate, since direct fetal measurement is often considered more reliable than cycle-based calculation alone.
How is due date calculated for irregular cycles?
For irregular cycles, LMP-based calculation using Naegele’s Rule becomes less reliable, making early ultrasound dating the preferred method, since it measures the fetus directly rather than relying on assumptions about ovulation timing.
What percentage of babies are born on their exact due date?
Only a small percentage of babies, generally cited around 4–5%, are actually born on their exact calculated due date, with the majority arriving within a week or two before or after that estimated date.
Is the due date calculated from conception or from the last period?
Standard due date calculation (Naegele’s Rule) uses the first day of the last menstrual period, not the conception date, even though pregnancy itself technically begins at conception, which is roughly two weeks after LMP in a typical cycle.