A pregnancy due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period. If your last period started on January 1, for example, your estimated due date would be October 8. This gives you a target, but it’s an estimate. Only about 68% of babies arrive within 11 days of that date, and the actual day of birth depends on factors unique to you.
How the 280-Day Calculation Works
The standard formula, known as Naegele’s rule, works like this: take the first day of your last menstrual period, add 7 days, then count forward 9 months. So if your last period started March 10, you’d add 7 days to get March 17, then move forward 9 months to arrive at December 17.
This formula assumes a 28-day menstrual cycle with ovulation happening on day 14. If your cycles are longer or shorter than 28 days, the estimate shifts. A 35-day cycle, for instance, means you likely ovulated a week later than the formula assumes, pushing your real due date about a week further out. If you know your cycle length, you can adjust by adding or subtracting the number of days your cycle differs from 28.
When Ultrasound Changes Your Due Date
A first-trimester ultrasound, done before 14 weeks, is the most accurate way to confirm or adjust a due date. During this scan, the technician measures the embryo from head to rump. At 8 weeks, that measurement is roughly 57 to 62 millimeters. By 12 weeks, it’s around 84 to 90 millimeters. These measurements correspond to specific gestational ages with an accuracy of plus or minus 5 to 7 days.
If the ultrasound date and your period-based date are close, your original estimate stands. But if there’s a meaningful gap between the two, your provider will typically use the ultrasound date instead. A second-trimester ultrasound (14 to 22 weeks) can also estimate gestational age, but the margin of error widens to 7 to 10 days, so earlier scans are preferred for dating.
Due Dates After IVF
If you conceived through IVF or another assisted method, the calculation is more precise because the date of conception is known. You add 266 days to the conception date. For embryo transfers, conception is calculated by subtracting the embryo’s age from the transfer date. A Day 5 blastocyst transferred on June 10, for instance, has a conception date of June 5, making the due date February 25 (266 days later). A Day 3 embryo transferred on the same date would have a conception date of June 7.
How Precise a Due Date Really Is
A due date is a midpoint in a range, not a deadline. The biology of when labor starts varies from person to person. Research tracking pregnancies from ovulation to delivery found that first-time mothers have a median pregnancy length of 274 days from ovulation, which is 8 days longer than the textbook 266 days. Women who have given birth before tend to deliver a bit sooner, with a median of 269 days from ovulation.
In practical terms, about half of first-time mothers will go into labor on their own by 40 weeks and 5 days. For mothers who have given birth before, that midpoint is 40 weeks and 3 days. So if you’re a first-time parent still pregnant at 40 weeks, you’re right on schedule, not late.
What “Full Term” Actually Means
Not all weeks near the end of pregnancy are treated the same. The American College of Obstetricians and Gynecologists breaks late pregnancy into distinct categories:
- Early term: 37 weeks 0 days through 38 weeks 6 days
- Full term: 39 weeks 0 days through 40 weeks 6 days
- Late term: 41 weeks 0 days through 41 weeks 6 days
- Postterm: 42 weeks 0 days and beyond
These labels matter because babies born at 39 weeks and beyond generally have better outcomes than those born at 37 or 38 weeks. The final weeks of pregnancy allow for important brain and lung development. A baby born at 37 weeks is not premature, but it’s not considered fully term either.
Quick Way to Estimate Right Now
If you know the first day of your last period, count forward 40 weeks on a calendar, or simply find the date 280 days later. If you don’t remember the exact date but know roughly when you conceived, add 266 days (38 weeks) from that date instead. Either method gets you to the same window. Your first ultrasound will either confirm that estimate or nudge it by a few days.