How is the Due Date Calculated? The Math of Pregnancy
Content reviewed against ACOG Committee Opinion No. 700 on estimating due dates.
Last reviewed: August 2026. This information is for educational purposes only. Always consult your healthcare provider.
One of the most confusing parts of early pregnancy is understanding how weeks are counted. If pregnancy is 9 months, why does everyone say it's 40 weeks? (Since 40 weeks is technically closer to 10 months!).
Furthermore, why does your doctor say you are "4 weeks pregnant" on the exact day you missed your period—when you likely only conceived 2 weeks ago?
Let's break down the clinical math behind how pregnancy weeks and due dates are actually calculated.
The LMP Rule (Last Menstrual Period)
Historically, before ultrasound technology existed, it was impossible to know exactly when ovulation or conception occurred. However, women almost always knew the date their last period started.
Because of this, the medical community standardized pregnancy tracking based on the first day of your Last Menstrual Period (LMP).
The 40-Week Math
By medical standards, a full-term pregnancy lasts 280 days (or exactly 40 weeks) starting from the first day of your LMP.
This means that during Week 1 and Week 2 of your "pregnancy," you are not actually pregnant yet! Your body is simply preparing for ovulation. Conception typically occurs at the end of Week 2.
Naegele's Rule
If you don't have a calculator handy, doctors use a simple formula called Naegele's Rule to estimate the Estimated Date of Delivery (EDD):
Step 1: Take the first day of your last period.
Step 2: Add 7 days.
Step 3: Subtract 3 months.
Example: If your LMP was November 1st, adding 7 days gives you November 8th. Subtracting 3 months gives you August 8th as your due date.
What if My Cycle Isn't 28 Days?
The standard LMP calculation assumes a perfect 28-day cycle with ovulation occurring exactly on day 14. If you have longer or shorter cycles, Naegele's Rule becomes slightly inaccurate.
This is why ultrasound dating is the gold standard in modern obstetrics.
The Ultrasound "Dating Scan"
Between 8 and 12 weeks of pregnancy, you will likely have a dating ultrasound. The sonographer will measure the fetus from the top of its head to its bottom (Crown-Rump Length, or CRL).
Because early fetal growth is incredibly uniform across all pregnancies, the CRL provides a highly accurate estimate of exactly how many weeks pregnant you are. If the ultrasound due date differs from your LMP due date by more than 5-7 days, your doctor will usually change your official due date to match the ultrasound.
Is the Due Date a Guarantee?
No! The Estimated Date of Delivery (EDD) is exactly that—an estimate. It simply marks the 40-week point. Only about 4% to 5% of babies are actually born on their exact due date.
A pregnancy is considered "full-term" anywhere between 37 weeks and 42 weeks, giving you a 5-week window in which it is perfectly normal and healthy for your baby to arrive.