Every pregnancy announcement includes two facts: the due date and the disbelief that it's so specific. "March 14th? But how do they know?" The honest answer: they don't know — they estimate, using one of the oldest formulas in medicine plus a modern correction from ultrasound. Here's exactly how that date is built.

An expectant mother silhouette with a weekly calendar

Naegele's rule: the 280-day standard

The default due date comes from Naegele's rule, published in 1812 and still the backbone of obstetrics:

Due date = first day of last menstrual period + 280 days (40 weeks)

Why 280? Because it's the average length of a pregnancy counted from the last menstrual period (LMP) — even though conception happens roughly two weeks after LMP. So "week 6 of pregnancy" means about 4 weeks since conception. Confusing? Everyone agrees, but the convention stuck because most people remember their last period better than their conception day.

Quick estimate: add 1 year to the LMP date, subtract 3 months, add 7 days. LMP of June 10, 2026 → due around March 17, 2027.

The ultrasound correction

Naegele's rule assumes a textbook 28-day cycle with ovulation on day 14 — but real cycles range from 21 to 35+ days, and ovulation follows the cycle, not the calendar. A shorter cycle means the due date is earlier than the rule predicts; a longer cycle means later.

That's why the first-trimester ultrasound is the gold standard: measuring the baby (crown–rump length) dates the pregnancy to within about 5–7 days, and the due date is usually adjusted to match it. Later ultrasounds are less precise at dating — babies, like people, grow at different rates.

Why weeks, not months

Obstetrics counts gestational age in weeks and days ("8w4d") because fetal development is week-by-week specific:

• Weeks 1–12 — organ formation (the first trimester)
• Weeks 13–27 — rapid growth, viability milestones approach
• Weeks 28–40 — lungs mature, baby gains the final weight

"How many months is 26 weeks?" has two answers depending on how you count — which is why doctors stick to weeks.

Trimesters and key milestones

• First trimester: weeks 1–13
• Second: weeks 14–27
• Third: 28 to birth

A due date also unlocks every other milestone: when first scans happen, when screening tests are offered, maternity leave planning, baby shower logistics. One date organizes the entire journey.

The fine print nobody should skip

1. A due date is a median, not a deadline. Only about 4–5% of babies arrive on it; roughly 60% come within a week of it. Full term is anywhere from 37 to 42 weeks.
2. "Overdue" is normal territory. Many first babies arrive at 40–41 weeks. Providers monitor, not panic.
3. IVF dates are exact — because the conception date is known, IVF pregnancies skip the LMP estimation entirely.

Estimate your own timeline:

Want to count another important timeline precisely? See How Old Are You, Really? for date math that's exact.