The most common way to figure out your due date is to count 280 days (40 weeks) from the first day of your last menstrual period. This gives you an estimated due date, not a guaranteed one. About 70% of babies arrive before that date, and only a small fraction are born on the exact day predicted. Still, having a reliable estimate matters for tracking your baby’s growth and planning your prenatal care.
The Standard Calculation From Your Last Period
The most widely used method is called Naegele’s Rule. Johns Hopkins Medicine breaks it into three steps:
- Start with the first day of your last menstrual period (LMP).
- Count back three calendar months from that date.
- Add one year and seven days.
So if your last period started on March 10, you’d count back to December 10 and then add a year and seven days, landing on December 17 as your estimated due date. This formula assumes a 28-day menstrual cycle with ovulation happening on day 14. It’s the same math behind every online due date calculator you’ll find.
What If Your Cycle Isn’t 28 Days?
A “normal” cycle can range from 21 to 35 days, and plenty of people fall outside even that window. If your cycle runs longer than 28 days, you likely ovulated later than day 14, which means the standard calculation will set your due date too early. The reverse is true for shorter cycles.
If you know your typical cycle length, you can adjust by adding or subtracting the difference. For example, if your cycle is 35 days, ovulation probably happened around day 21 instead of day 14, pushing your actual due date about a week later than Naegele’s Rule suggests. In practice, though, most providers rely on an early ultrasound to sort this out rather than asking you to do the math yourself.
If You Know Your Ovulation or Conception Date
When you know the exact day you ovulated or conceived, whether from tracking with ovulation kits, basal body temperature, or fertility treatment, the calculation is simpler: add 266 days to your conception date. That’s 38 weeks, which is the same 40-week total minus the two weeks before ovulation that the LMP method includes. This approach tends to be more accurate because it removes the guesswork about when you actually ovulated.
How IVF Due Dates Are Calculated
If you conceived through IVF, your due date is calculated from either the egg retrieval date or, more commonly, the embryo transfer date. For a blastocyst transfer (day 5, 6, or 7 embryo), the formula works backward: conception date equals the transfer date minus the embryo’s age in days. From there, you add 266 days. A day-3 embryo transfer uses the same logic, just with a different embryo age subtracted.
Because IVF pinpoints the exact timing of conception, these due dates are typically the most precise of any method. Your fertility clinic will calculate this for you, and there’s rarely a reason to adjust it later.
How Ultrasound Refines Your Due Date
An early ultrasound is the most reliable way to confirm or correct a due date. During the first trimester, a technician measures the embryo from head to rump. Before 9 weeks, this measurement is accurate to within about 5 days. Between 9 and 14 weeks, the margin widens to roughly 7 days. The earlier the ultrasound, the more precise it is, because embryos grow at very predictable rates in those first weeks. Later in pregnancy, babies grow at increasingly individual rates, making size a less reliable indicator of age.
The American College of Obstetricians and Gynecologists has clear rules for when an ultrasound should override your period-based date. Before 9 weeks, if the ultrasound disagrees with the LMP calculation by more than 5 days, the ultrasound date wins. Between 9 and 14 weeks, the threshold is 7 days. If you’re uncertain about your last period at all, dating should be based on the earliest ultrasound available, ideally before 14 weeks.
Why Your Due Date Might Change
It’s common to have your due date shifted after an early ultrasound, and it doesn’t mean anything is wrong. The most frequent reason is simply that ovulation didn’t happen on day 14 of your cycle. Irregular periods, recent use of hormonal birth control, and breastfeeding while conceiving can all make the LMP date unreliable. A first-trimester ultrasound that disagrees with your period-based date by more than a few days is enough for your provider to update it.
Once a due date is established by early ultrasound, later ultrasounds generally won’t change it. A baby measuring large or small in the second or third trimester reflects individual growth patterns, not a dating error.
What “Full Term” Actually Means
Your due date marks 40 weeks, but healthy babies arrive across a wide window. The National Institutes of Health defines the categories like this:
- Early term: 37 weeks through 38 weeks, 6 days
- Full term: 39 weeks through 40 weeks, 6 days
- Late term: 41 weeks through 41 weeks, 6 days
- Post-term: 42 weeks and beyond
About 10% of deliveries happen before 37 weeks (preterm), and roughly 70% of all babies arrive before the due date itself. So while the due date gives you a target, think of it as the center of a range rather than a deadline. Most providers consider anything between 39 and 41 weeks to be right on time.