Your due date is not a prediction carved in stone. It is a calculation, built from a simple rule invented in the 1800s, refined by ultrasound, and still wrong for most people by a week or more. Here is how the math actually works, and why that is normal.
Naegele's rule: the 280-day calculation
Most due dates start with Naegele's rule, named after the German obstetrician Franz Karl Naegele. The rule is simple: take the first day of your last menstrual period, add one year, subtract three months, and add seven days. In practice, that is the same as adding 280 days, or 40 weeks, to that first day.
Why 280 days? A typical menstrual cycle is about 28 days, and ovulation usually happens around day 14. Counting from the last period rather than from conception adds roughly two weeks before conception even occurs, which is why pregnancy is dated at 40 weeks even though the embryo is only about 38 weeks old at term. It is a convention, and every pregnancy calendar, appointment schedule, and trimester boundary uses it.
The rule assumes a regular 28-day cycle. If your cycles are reliably longer or shorter, the rule alone will be off, which is why a first-trimester ultrasound often revises the date. More on that below.
Worked example: counting forward 280 days
Suppose the first day of the last menstrual period was January 1. Add 280 days and you land on October 8. Using the shortcut version: add one year (January 1 next year), subtract three months (October 1), and add seven days (October 8). Same answer.
Try another: a last period starting March 10 gives a due date of December 17. March 10 plus one year is March 10 next year, minus three months is December 10, plus seven days is December 17. You can check the same result by counting 40 weeks forward on a calendar, week by week.
That date is called the estimated due date, and the word estimated does real work. It marks the middle of a window, not a deadline. Babies born between 37 and 42 weeks are all considered within the normal range, and only a small minority arrive on the estimated day itself.
Why only about 5 percent arrive on the day
The often-quoted figure is that only around 5 percent of babies are born on their due date. That sounds alarming until you understand what the date represents. Human gestation varies naturally: cycle length varies, ovulation timing varies, implantation timing varies, and the length of pregnancy itself varies from person to person by weeks.
Think of the due date as the peak of a bell curve, not a finish line. Roughly speaking, many births cluster in the two weeks before and after it, with a long tail on either side. A baby born at 39 weeks and a baby born at 41 weeks are both perfectly ordinary outcomes of the same calculation. The date is useful for planning checkups, tests, and leave, not for judging whether a pregnancy is on schedule day by day.
It also helps to know the labels clinicians use. Before 37 weeks is preterm, 37 to 38 weeks is early term, 39 to 40 weeks is full term, 41 weeks is late term, and 42 weeks and beyond is postterm. Crossing the due date by a few days does not change any of those categories.
Last period dating versus ultrasound dating
When the two methods disagree, which one wins? In early pregnancy, ultrasound usually wins. A first-trimester ultrasound measures the embryo, and at that stage embryos grow at a remarkably consistent rate, so the measurement dates the pregnancy to within a few days.
Clinicians typically follow a practical rule: if the ultrasound date and the last-period date differ by more than about five to seven days in the first trimester, the ultrasound date replaces the last-period date. Later ultrasounds are less precise for dating because babies start growing at different rates, so a second- or third-trimester scan is a poor tool for resetting the date.
- Regular 28-day cycles: last-period dating and early ultrasound usually agree within a few days.
- Irregular or long cycles: last-period dating can be off by two weeks or more, and ultrasound dating matters much more.
- Recent hormonal birth control or breastfeeding: periods may be unreliable markers, so ultrasound dating carries extra weight.
- IVF pregnancies: dating is exact, because the conception date is known. The due date is calculated from the transfer or retrieval date instead.
If your due date was revised after a scan, that is not a sign something went wrong. It is the system working as designed: an early measurement replacing an assumption.
What the due date is actually for
A due date organizes care. It sets the schedule for screening tests, which are timed to specific weeks, and it tells clinicians when to start watching more closely for postterm concerns. It also gives families a planning anchor for work leave, childcare, and hospital logistics.
What it is not for is a day-by-day verdict. Being a week past the date does not mean the baby is overdue in a dangerous sense, and being a week early does not mean the baby is premature. The clinical decisions at the end of pregnancy depend on monitoring, the baby's condition, and the mother's health, not on the calendar alone.
A practical approach: prepare for a window, not a day. Have the bag packed and the plan ready by 37 weeks, expect the birth anywhere from 37 to 42 weeks, and treat the estimated date as the center of that window. That mindset removes most of the anxiety the number itself creates.
A quick checklist for your own date
- Note the first day of your last menstrual period, not the day it ended.
- Add 280 days, or use the add-a-year, subtract-three-months, add-seven-days shortcut.
- Compare with your first-trimester ultrasound if you had one, and use the ultrasound date if it differs by more than a few days.
- Remember the window: 37 to 42 weeks is the normal range, and only a small share of births land on the estimated day.
- Ask your clinician which date is on your chart, so every appointment and test uses the same one.
One number, honestly understood, is far more useful than a precise-sounding date taken literally. The math is simple, the biology is variable, and both facts can be true at once.
Frequently asked questions
Is a due date exact?
No. A due date is an estimate marking the middle of a normal window, roughly 37 to 42 weeks. Only a small minority of babies, often quoted around 5 percent, are born on the estimated day itself. Births a week or two on either side are entirely ordinary.
Can my due date change after an ultrasound?
Yes, and it often does. If a first-trimester ultrasound date differs from the last-period date by more than about five to seven days, clinicians usually adopt the ultrasound date. Later scans are less reliable for dating, so the date is rarely revised after the first trimester.
What if my periods are irregular?
Irregular cycles make last-period dating unreliable, sometimes by two weeks or more, because ovulation may not fall on day 14. In that case, an early ultrasound is the better dating tool. Tell your clinician your cycles are irregular so the scan is weighted accordingly.
How is a due date calculated for IVF?
For IVF, dating is exact because the conception date is known. The due date is calculated from the egg retrieval or embryo transfer date rather than from a last menstrual period. This removes the cycle-length guesswork that affects natural-conception dating.