Pregnancy due dates are calculated by counting 280 days (40 weeks) from the first day of your last menstrual period. Only about 5% of babies actually arrive on that date, but it serves as the central reference point for prenatal care, growth tracking, and delivery planning. Here’s how the math works and why your due date might shift.
The Standard Formula
The most widely used method is called Naegele’s rule, a simple three-step calculation. 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 June 10, 2025, you’d count back to March 10, then add a year and seven days to land on March 17, 2026.
This formula assumes a 28-day menstrual cycle with ovulation occurring on day 14. That’s a clean assumption, but it doesn’t reflect everyone’s biology. The 280-day total includes roughly two weeks before you actually conceived, because the clock starts at your last period rather than at fertilization. This is why pregnancy is described as 40 weeks even though the embryo is only about 38 weeks old at birth.
Why Gestational Age Starts Before Conception
Gestational age and the actual age of the embryo are two different numbers. Gestational age is measured from the first day of your last period, which typically adds about two weeks to the embryo’s true biological age. So when you’re told you’re “8 weeks pregnant,” the embryo has been developing for closer to 6 weeks.
This system exists because most people can identify when their last period started far more reliably than they can pinpoint the exact day of ovulation or conception. It’s a practical convention, not a biological one, and it’s baked into every prenatal milestone and growth chart your provider uses.
How Ultrasound Refines the Estimate
A first-trimester ultrasound is the most accurate tool for confirming or adjusting a due date. Between about 6 and 14 weeks, a measurement called the crown-rump length (the distance from the top of the embryo’s head to the bottom of its torso) can estimate gestational age within 5 to 7 days. That’s a tighter margin than most period-based calculations can offer.
After 14 weeks, this particular measurement becomes less reliable because embryos start growing at more individual rates. Second-trimester ultrasounds use other measurements, like head circumference and thigh bone length, but the margin of error widens. This is why early ultrasounds carry the most weight when establishing a due date. If there’s a significant discrepancy between what your last period suggests and what the ultrasound shows, your provider will typically go with the ultrasound dating from that early scan.
When Your Cycle Isn’t 28 Days
Naegele’s rule loses accuracy when your cycle deviates from 28 days. If your cycle runs longer, say 35 days, you likely ovulated later than day 14, which means your actual due date is later than the formula predicts. Shorter cycles push it earlier. For someone with a 21-day cycle, ovulation probably happened around day 7 rather than day 14, shifting the real conception date by a full week.
Irregular cycles make things even less predictable. If your cycle fluctuates between 21 and 35 days from month to month, a period-based calculation can be off by a week or more in either direction. This is one of the main reasons early ultrasound dating is so valuable. It bypasses the cycle-length question entirely and measures the pregnancy directly.
Due Dates After IVF or Frozen Embryo Transfer
Pregnancies conceived through IVF have one major advantage when it comes to dating: the exact moment of fertilization (or very close to it) is known. The calculation uses the date of the embryo transfer rather than a last menstrual period. For frozen embryo transfers at the blastocyst stage, the due date calculation is the same whether the embryo was frozen on day 5, day 6, or day 7, since they’re all transferred at a comparable developmental stage.
This makes IVF due dates inherently more precise than those calculated from menstrual history, where ovulation timing is always an estimate.
Factors That Affect Actual Delivery Timing
Even a perfectly calculated due date is a statistical midpoint, not a deadline. Several biological factors influence how long a pregnancy actually lasts. Research published in the American Journal of Obstetrics & Gynecology identified maternal age, whether you’ve given birth before, and race as the most significant variables.
First-time mothers tend to carry slightly longer, with an average gestational length of about 276 days compared to roughly 274.5 days for those who’ve delivered before. That difference is small but consistent across large datasets. Women under 19 or over 34, along with Black women, tend to have somewhat shorter gestations on average. These patterns don’t change your official due date, but they’re worth knowing when you’re wondering whether your baby is “late.”
A full-term pregnancy spans 273 to 286 days, which translates to a window of about 37 to 42 weeks. Delivery anywhere in that range is considered normal. The 280-day target is simply the center of that window.
How Your Provider Tracks Progress
Beyond the initial dating, providers use a simple physical check to monitor growth throughout the second half of pregnancy. After 24 weeks, the distance from your pubic bone to the top of your uterus (measured in centimeters with a tape measure) roughly matches the number of weeks pregnant you are, give or take about 3 centimeters. At 30 weeks, for example, this measurement should be somewhere around 27 to 33 centimeters. A number that falls well outside that range can prompt further evaluation with ultrasound, but on its own, it’s just a quick screening tool.
What the 5% Number Really Means
Only about 5 out of 100 people give birth on their estimated due date. That number surprises most people, but it makes sense when you consider what the due date actually represents. It’s the single most likely day for delivery, but “most likely” in this case still means a 95% chance of delivering on a different day. The realistic window is several weeks wide.
Thinking of your due date as the center of a two-to-three-week range is more accurate than treating it as a firm target. Babies born anywhere between 37 and 42 weeks are considered term. Planning and mental preparation tend to go better when you expect that range rather than circling one specific date on the calendar.