How to calculate your due date
May 4, 2026 · 7 min read
The first thing almost everyone wants to know after a positive pregnancy test is "when is the baby coming?" The math behind that answer is older than most countries on Earth, and it gets one fundamental thing slightly wrong on purpose. Here's how due date calculation actually works — and why your first prenatal ultrasound might quietly rewrite it.
The 280-day rule
The standard formula goes by the name of Naegele's rule, after the German obstetrician Franz Karl Naegele who published it in his 1812 textbook. The rule:
Due date = first day of last menstrual period (LMP) + 280 days.
That's exactly 40 weeks. Or, if you prefer the popular shortcut: add seven days to the LMP, then count back three months and add one year. Same arithmetic, less subtraction.
Wait — why count from the period?
This is the bit that confuses everyone. Pregnancy is measured from the first day of your last period, not from conception. Two reasons:
- It's knowable. Most people know exactly when their last period started. Far fewer know exactly when they conceived.
- It standardizes care. Every clinician uses the same convention, so "you're 12 weeks pregnant" means the same thing in every office.
The downside is that the first two "weeks" of pregnancy aren't really pregnancy at all — you haven't even ovulated yet. By the time most people get a positive test, they're typically called four weeks pregnant. Bizarre, but consistent.
The cycle-length adjustment
Naegele's rule assumes a 28-day cycle with ovulation on day 14. About one-third of women have cycles that long; the rest have shorter or longer ones. If your cycle is 32 days, your ovulation — and therefore your conception — is roughly four days later than the textbook assumes. So your due date is four days later too.
The adjusted formula:
Due date = LMP + 280 + (cycle length − 28).
Most online due date calculators (including ours) make this adjustment automatically.
The conception-date method
If you actually know when you conceived — IVF, fertility tracking with confirmed ovulation, a single well-timed cycle — the math is more direct:
Due date = conception + 266 days.
That's 280 minus the 14-day lead time from LMP to ovulation in the standard convention. Same answer, simpler input.
The ultrasound that might change everything
Here's the plot twist: your due date may shift after your first prenatal visit. Most providers do a dating ultrasound in the first trimester, ideally before 13 weeks. They measure the embryo's crown-rump length — a remarkably consistent indicator of gestational age in early pregnancy.
If the ultrasound dating disagrees with your LMP-based calculation by more than about a week, the ultrasound usually wins. Why? Because cycle irregularities, mistimed ovulation, and forgotten period dates introduce more error than crown-rump-length measurement does.
This is good news. It means your due date will eventually be as accurate as a clinical estimate can be — you don't have to nail the LMP date down to the day.
The most important thing to remember
Only about 5% of babies are born on their actual due date. About 80% arrive within two weeks before or after. A due date is a median estimate, not a deadline. It exists to plan prenatal care, schedule screening, and decide if your provider should consider induction once you pass 41-42 weeks. Beyond that, it's a number to share with your boss when planning leave.
Babies arrive when they arrive.