A pregnancy due date is calculated by counting 280 days (40 weeks) from the first day of your last menstrual period. That number assumes a 28-day cycle and that ovulation happened on day 14, which means the calculation includes roughly two weeks before you actually conceived. Only about 5% of babies arrive on their due date, so think of it as an estimate rather than an appointment.
The Standard Formula: Naegele’s Rule
The most common method for calculating a due date is called Naegele’s rule. Here’s how it works:
- Start with the first day of your last menstrual period (LMP).
- Count back three calendar months.
- Add one year and seven days.
So if your last period started on March 10, you’d count back to December 10, then add a year and seven days to get December 17. This formula has been the standard in obstetrics for over a century, and it’s what most online due date calculators use behind the scenes.
The catch is that Naegele’s rule assumes your cycle is exactly 28 days long and that you ovulated on day 14. Many people don’t fit that mold. If your cycles run longer or shorter than 28 days, a simple adjustment helps: add or subtract the number of days your cycle differs from 28. For example, if your cycle is typically 35 days, you’d add 7 days to the standard due date, since you likely ovulated a week later than the formula assumes.
Why You’re “Pregnant” Before Conception
One of the most confusing things about pregnancy dating is that it starts counting before you’ve conceived. Ovulation doesn’t happen until about two weeks into your cycle, and fertilization occurs around that time. But because the first day of your last period is easier to pinpoint than the exact day of ovulation, the medical system uses it as the starting line.
This is why a positive pregnancy test taken two weeks after ovulation already puts you at “four weeks pregnant.” You weren’t actually pregnant for those first two weeks. The 40-week timeline is really about 38 weeks of actual fetal development plus two weeks of pre-conception cycle time.
How Ultrasound Changes the Date
An early ultrasound is the most accurate way to confirm or adjust a due date. During the first trimester (up to about 13 weeks and 6 days), the technician measures the baby from the top of the head to the bottom of the spine. At this stage, embryos grow at a remarkably predictable rate, so that measurement can pin down gestational age to within 5 to 7 days.
That precision matters. In one study, 40% of women who received a first-trimester ultrasound had their due date changed because it differed from the period-based estimate by more than five days. Among women who didn’t get an ultrasound until the second trimester, only 10% needed an adjustment. First-trimester dating is more reliable because early growth is highly uniform. Later in pregnancy, babies start growing at different rates based on genetics, nutrition, and other factors, which makes size a less trustworthy indicator of age.
If your ultrasound date and your period-based date are close, your provider will typically stick with the original estimate. When there’s a meaningful gap, the ultrasound date takes priority, especially if it was done early.
Due Dates After IVF
For pregnancies conceived through in vitro fertilization, the calculation is more straightforward because the exact date of fertilization is known. The due date is typically counted from the date of embryo transfer, and the math is adjusted based on how developed the embryo was at transfer. Because there’s no guesswork about when ovulation happened, IVF due dates skip the two-week buffer that’s built into natural conception calculations. Whether a frozen embryo was transferred on day 5, 6, or 7 of development, the due date calculation treats them the same since they’re all at the same developmental stage.
Factors That Shift When You’ll Actually Deliver
Even a perfectly calculated due date is just an estimate of the middle of a normal delivery window. Several biological factors influence how long a pregnancy actually lasts. Whether you’ve given birth before is one of the biggest: people who have had previous pregnancies tend to deliver slightly earlier than first-time parents. Maternal age also plays a role. Those younger than 19 or older than 34 tend to have somewhat shorter pregnancies compared to those in the 19-to-34 range. Race and ethnicity affect average gestation length as well.
These variables are why some researchers have proposed more personalized formulas that account for individual characteristics rather than applying one rule to everyone. Naegele’s rule gives you a single point on the calendar, but the reality is that a healthy, full-term baby can arrive anywhere within a few weeks of that date. Most providers consider 37 to 42 weeks the normal range for delivery, and the due date simply marks the 40-week midpoint.
Why Getting the Date Right Matters
An accurate due date isn’t just about planning your hospital bag. It guides medical decisions throughout pregnancy. Certain screening tests need to happen within specific gestational windows to be reliable. If your dates are off by even a week or two, a test result might be misinterpreted. Later in pregnancy, your provider uses the due date to determine whether labor needs to be induced or whether the baby is growing on track. A date that’s too early could make a normal pregnancy look overdue, leading to unnecessary interventions. A date that’s too late could delay care when the pregnancy has genuinely gone past term.
This is a big part of why early ultrasound dating has become standard practice. Even when you’re confident about the date of your last period, an early scan can catch discrepancies that would otherwise ripple through months of prenatal care.