A due date is a single calendar day, which makes it sound far more precise than it is. Roughly 1 in 20 babies actually arrive on theirs.
Understanding where the number comes from — and what it quietly assumes about you — makes it a lot easier to hold loosely.
Naegele's rule
The standard calculation dates back to a 19th-century German obstetrician and is still what most due dates are based on:
280 days is 40 weeks. The arithmetic shortcut clinicians use:
Example: last period began 10 January 2026.
- Subtract 3 months → 10 October 2025
- Add 7 days → 17 October 2025
- Add 1 year → 17 October 2026
Why it's counted from your period, not conception
This is the part that confuses nearly everyone, and it's genuinely a bit strange.
Pregnancy is dated from the first day of your last period — which is before conception. In a textbook 28-day cycle, ovulation happens around day 14, so you're already counted as "2 weeks pregnant" at the moment of conception.
The reason is purely practical: most people know roughly when their last period started. Almost nobody knows the exact day conception occurred, and sperm can survive several days in the reproductive tract, so even a known date of intercourse doesn't pin it down.
So gestational age runs about two weeks ahead of the embryo's actual age. A "40-week" pregnancy involves roughly 38 weeks of actual development.
The 28-day assumption
Naegele's rule assumes a 28-day cycle with ovulation on day 14. Plenty of people don't have that.
If your cycles run consistently longer or shorter, the estimate shifts, because what actually varies is the first half of the cycle. The luteal phase — ovulation to period — is fairly stable at around 14 days for most people. A longer cycle generally means later ovulation, not a longer pregnancy.
On a 35-day cycle that's 7 days later; on a 24-day cycle, 4 days earlier. Worth raising with your provider rather than adjusting on your own, since they may prefer to date from a scan instead.
Estimate your due date from your last period
Open Due Date Calculator →Ultrasound dating
An early ultrasound is generally more reliable than period-based dating, and if the two disagree by more than a certain margin, the scan usually wins.
In the first trimester, embryos grow at a remarkably consistent rate, so measuring crown-rump length gives a tight estimate — often accurate to within about five days. That consistency is what makes early scans so useful for dating.
Later scans are much weaker for dating. By the third trimester, genuine variation in size has taken over: some babies are simply bigger or smaller than average, and a measurement can't distinguish "large for dates" from "further along than we thought." This is why dating is normally locked in early and not revised later, even if growth measurements drift.
If you conceived through IVF, the dates are known precisely from the transfer, and that takes precedence over everything else.
What the term categories mean
"Term" used to cover everything from 37 to 42 weeks as if it were uniform. It was later split up, because outcomes differ measurably across that range:
| Classification | Gestational age |
|---|---|
| Preterm | Before 37 weeks |
| Early term | 37w 0d – 38w 6d |
| Full term | 39w 0d – 40w 6d |
| Late term | 41w 0d – 41w 6d |
| Post-term | 42w 0d and beyond |
The practical upshot is that 37 weeks is no longer treated as equivalent to 39. Babies born at 37–38 weeks have somewhat higher rates of breathing difficulties and feeding problems than those born at 39–40, which is part of why elective deliveries before 39 weeks are generally discouraged without a medical reason.
Holding the date loosely
Most births cluster in the couple of weeks around the due date rather than landing on it. First pregnancies tend to go slightly longer than subsequent ones.
A few things that help:
- Think in weeks, not a day. "Due in late October" is a more honest description than a specific date, and saves you a fortnight of daily phone calls asking whether anything's happened.
- Plan leave with a buffer. Arriving two weeks early is entirely normal, not an emergency.
- Expect a conversation about 41 weeks. Monitoring or induction is commonly discussed once you pass the due date, since risks rise gradually afterward. The specifics depend on your pregnancy and your provider.
Related: Pregnancy Calculator for week-by-week milestones, and the Ovulation Calculator for estimating a fertile window.