A due date is the estimated day your baby will be born, calculated as 40 weeks (280 days) from the first day of your last menstrual period. It’s also called an estimated due date or EDD. Despite the name, only about 5% of babies actually arrive on this exact date. It’s best understood as the center of a window rather than a deadline.
How Your Due Date Is Calculated
The most common method is called Naegele’s rule, and it’s straightforward. Take the first day of your last menstrual period, count back three calendar months, then 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 assumes a 28-day menstrual cycle with ovulation happening around day 14.
If your cycle is shorter or longer than 28 days, this calculation becomes less reliable. Someone with a 35-day cycle ovulates later, which means their actual conception happened later than Naegele’s rule assumes. The same goes for irregular cycles. In these cases, an ultrasound is typically a better way to pin down the date.
Why the Count Starts Before Conception
One thing that confuses many people: you aren’t actually pregnant for the first two weeks of your “pregnancy.” The 40-week count begins on the first day of your last period, but conception doesn’t happen until ovulation, roughly two weeks later. This means your baby’s actual age is about two weeks less than the gestational age your provider uses. When your doctor says you’re 12 weeks pregnant, the embryo has been developing for closer to 10 weeks. This two-week gap is just a quirk of the dating system, since the start of your last period is a date most people can identify with certainty, while the exact day of conception is harder to know.
Ultrasound Dating and When It’s More Accurate
Early ultrasounds are the most precise way to estimate a due date. During the first trimester, the technician measures the baby from head to rump (called a crown-rump length). Before 14 weeks, this measurement can pin down gestational age to within five to seven days. The earlier in the first trimester the scan is done, the more accurate it is.
After 14 weeks, the baby’s growth rate becomes more variable from one pregnancy to the next, so later ultrasounds are less reliable for dating. Second-trimester scans use different measurements like head circumference and thigh bone length, but these carry a wider margin of error. This is one reason early prenatal care matters: an early scan gives the most trustworthy due date.
If your ultrasound date and your period-based date disagree by more than a few days, your provider will often go with the ultrasound, especially if it was done early.
How Likely You Are to Deliver on Your Due Date
About 5 in 100 people give birth on the exact day. Most babies arrive sometime in the two weeks before or after the due date. This is why doctors now use more specific language to describe the end of pregnancy:
- Early term: 37 weeks through 38 weeks and 6 days
- Full term: 39 weeks through 40 weeks and 6 days
- Late term: 41 weeks through 41 weeks and 6 days
- Post-term: 42 weeks and beyond
These categories replaced the older habit of calling everything from 37 to 42 weeks simply “term.” The distinction matters because babies born at 39 weeks tend to do better than those born at 37 weeks, even though both fall within what was once considered the normal range. The ideal window for delivery is that full-term range of 39 to 40 weeks.
What Happens If You Go Past Your Due Date
Going a few days or even a week past your due date is common and, on its own, not a cause for alarm. Your provider will typically start monitoring more closely around 41 weeks, often with twice-weekly checks that measure the baby’s heart rate patterns and the amount of amniotic fluid. These tests help confirm the baby is still doing well.
Pregnancies that extend beyond 42 weeks carry higher risks, including a greater chance of complications during delivery and, in rare cases, stillbirth. For this reason, induction of labor is often discussed between 41 and 42 weeks. Research shows that inducing in this window is associated with fewer complications compared to waiting for labor to start on its own past 42 weeks. The decision involves several factors, including your preferences, whether your body is showing signs of readiness for labor, and whether this is a first pregnancy.
First Pregnancy vs. Later Pregnancies
First-time parents tend to deliver slightly later than the due date on average, while people who have given birth before are somewhat more likely to deliver closer to or before it. Neither pattern is a rule, just a statistical tendency. Your own due date remains an estimate regardless of how many pregnancies you’ve had.
It’s worth thinking of your due date less as a specific day to circle on the calendar and more as the middle of a three-to-four-week stretch when labor could reasonably begin. Planning for a range rather than a single date can make those final weeks of waiting feel less stressful.