Your pregnancy due date is calculated by counting 280 days (40 weeks) from the first day of your last menstrual period. Only about 5% of babies arrive on that exact date, which is why it’s called an “estimated” due date. The calculation can come from a simple formula, an ultrasound, or both, and each method carries a different margin of error.
The Last Menstrual Period Method
The most common starting point is a formula called Naegele’s Rule. Johns Hopkins Medicine breaks it into three steps: take the first day of your last menstrual period, count back three calendar months, then add one year and seven days. If your last period started on March 10, for example, you’d count back to December 10, then add a year and seven days to land on December 17 of the following year.
This formula assumes two things: that your cycle is 28 days long and that you ovulated around day 14 of that cycle. For many people, those assumptions hold up reasonably well. But if your cycles run longer or shorter than 28 days, or if you ovulate later than day 14, the estimate can be off by a week or more. Women with irregular cycles are especially likely to see a gap between the formula’s prediction and what ultrasound later shows.
It’s also worth noting that gestational age counts from the start of your last period, not from conception. Since ovulation and fertilization typically happen about two weeks into a cycle, the actual age of the embryo is roughly two weeks younger than the “gestational age” your provider uses. That’s normal and intentional. The entire prenatal timeline, from growth charts to delivery planning, is built around this convention.
How Ultrasound Refines the Date
Ultrasound is the most reliable way to confirm or adjust a due date, and timing matters enormously. In early pregnancy, embryos grow at a remarkably predictable rate, so measuring the length of the embryo (from the top of the head to the bottom of the torso) gives a very precise age estimate. Before 9 weeks, a difference of more than 5 days between the ultrasound date and the period-based date is enough reason for your provider to change the due date.
As pregnancy progresses, babies start growing at more individual rates, influenced by genetics, nutrition, and other factors. That makes later ultrasounds less accurate for dating. Between 22 and 27 weeks, the margin of error widens to about 14 days. In the third trimester (28 weeks and beyond), ultrasound dating carries a margin of plus or minus 21 to 30 days, making it a poor tool for establishing a due date if one hasn’t already been set.
The American College of Obstetricians and Gynecologists considers any pregnancy that hasn’t had a dating ultrasound before 22 weeks to be “suboptimally dated.” This doesn’t mean something is wrong. It means providers have less confidence in the timeline, which can affect decisions later, such as whether labor should be induced for a pregnancy that seems to be going past its due date.
When Your Due Date Gets Changed
Providers don’t change a due date casually. Specific thresholds guide the decision, and they depend on how far along you are when the ultrasound is done. Before 9 weeks, a discrepancy of more than 5 days triggers a change. In the second trimester, the gap needs to be more than 14 days. In the third trimester, more than 21 days.
For discrepancies that fall in a gray zone, especially between 10 and 14 days in the second trimester, your provider weighs how confident you are about your last period date and how early in that trimester the ultrasound was performed. If you’re certain of your dates and the discrepancy is small, the original due date often stands. If your periods were irregular or you’re unsure of the timing, the ultrasound date takes priority.
Due Dates for IVF Pregnancies
If you conceived through IVF, your due date is actually more precise than in naturally conceived pregnancies because the exact date of fertilization or embryo transfer is known. The calculation starts from the date the embryo was transferred into the uterus rather than from a last menstrual period. For frozen embryo transfers at the blastocyst stage (the most common type), the due date calculation is the same whether the embryo was frozen on day 5, 6, or 7, since they’re all considered blastocyst-stage transfers.
Why So Few Babies Arrive on Time
A due date is a midpoint of a range, not a deadline. Only about 5 out of 100 babies are born on their exact due date. Most full-term births happen within a window of about two weeks on either side. Several biological variables contribute to this spread: the length of the follicular phase (the first half of the menstrual cycle) varies from person to person, implantation timing differs, and the hormonal signals that trigger labor don’t follow a universal clock.
The 40-week figure itself is a statistical average. Some healthy pregnancies naturally run 38 weeks, others 41. First-time pregnancies tend to run a bit longer. Your provider uses the due date to track growth, schedule screenings, and plan for delivery, but they expect flexibility around that number. A pregnancy is considered full-term anywhere from 39 weeks through 40 weeks and 6 days, with 37 to 38 weeks classified as “early term” and 41 weeks as “late term.”
What Affects Your Due Date’s Accuracy
The single biggest factor is how early in pregnancy the dating was done. An ultrasound at 7 weeks is accurate to within a few days. One at 30 weeks could be off by nearly a month. If you had both an early ultrasound and a period-based calculation that agree, your due date is about as reliable as it can get.
Cycle length is the second major variable. If your cycles are consistently 35 days instead of 28, ovulation likely happens around day 21 rather than day 14, pushing the true due date about a week later than Naegele’s Rule would suggest. Some providers adjust the formula for known cycle lengths, but many rely on the first ultrasound to catch these discrepancies instead.
Uncertainty about the last period is more common than you might think. Spotting in early pregnancy can mimic a period, leading to an inaccurate start date. Hormonal birth control used shortly before conception can also make the last “period” unreliable, since withdrawal bleeding on the pill isn’t a true menstrual cycle. In all of these cases, early ultrasound becomes the primary dating tool.