Pregnancy Calculator Due Date Accuracy Why It's 40 Weeks Not 9 Months

Someone tells you they're "15 weeks pregnant." You do the math: 15 weeks is about 3.5 months. But they conceived roughly 13 weeks ago. Why the 2-week gap? Because pregnancy is counted from the first day of the last menstrual period (LMP), not from conception — and that changes everything about how due dates work.

Pregnancy dating is one of those things that makes no sense until someone explains it, and then it makes perfect sense. Here's the system and why it exists.

Why 40 Weeks and Not 9 Months?

The average pregnancy from conception to birth is about 38 weeks (266 days). But since most people don't know their exact conception date, doctors count from the first day of the last menstrual period — which is typically about 2 weeks before ovulation and conception. That makes the "official" pregnancy length 40 weeks (280 days).

This system is called Naegele's rule, developed by German obstetrician Franz Naegele in 1812. He observed that adding 280 days (40 weeks) to the first day of the LMP predicted birth dates reasonably well for women with regular 28-day cycles. It's been the standard for over 200 years, despite its limitations.

Why not just use 9 months? Because calendar months vary in length (28-31 days). Nine calendar months could be anywhere from 273 to 276 days — not the 280 days of a standard pregnancy. And lunar months (28 days) give you 10 months, not 9. Weeks are consistent: always 7 days, no exceptions.

How Accurate Are Due Dates?

Not very. Only about 4-5% of babies are born on their exact due date. About 80% arrive within two weeks of the due date (38-42 weeks). The due date is best thought of as a due month — any time within about two weeks on either side is normal.

First-trimester ultrasound dating is more accurate than LMP dating, especially for women with irregular cycles. An ultrasound at 8-12 weeks measures the crown-rump length of the embryo, which grows at a very consistent rate in early pregnancy. This measurement can date a pregnancy within 5-7 days.

Factors that affect due date accuracy: irregular menstrual cycles (LMP method overestimates or underestimates), late ovulation, maternal age (older mothers tend to deliver slightly earlier), first vs subsequent pregnancies (first babies average 41 weeks + 1 day), and ethnicity (some studies show small but consistent differences in average gestation length).

The Trimester System

Pregnancy is divided into three trimesters of roughly 13 weeks each: First trimester (weeks 1-13): organ development, highest miscarriage risk. Second trimester (weeks 14-26): rapid growth, anatomy scan at 20 weeks. Third trimester (weeks 27-40): final growth, lung maturation, positioning for birth.

The trimester system matters because medical care, screening tests, and risk profiles change at each stage. It's not just a convenient way to divide time — it reflects distinct phases of fetal development.

For calculating your due date and tracking trimesters, use our pregnancy calculator with LMP or conception date input. For calculating the exact age after birth, our age calculator tracks years, months, and days. And for tracking healthy weight gain during pregnancy, our BMI calculator provides pre-pregnancy baseline measurements.