A pregnancy due date is calculated by adding 280 days (40 weeks) from the first day of your last menstrual period. Only about 5% of babies arrive on their exact due date, so think of it as an estimate rather than a deadline. There are several ways to calculate it, and each method carries a different level of accuracy.
The Standard Formula: Naegele’s Rule
The most common method, known as Naegele’s Rule, uses three simple steps:
- Step 1: Find the first day of your last menstrual period (LMP).
- Step 2: Count back three calendar months from that date.
- Step 3: Add one year and seven days.
For example, if your last period started on March 10, 2025, you’d count back three months to December 10, then add one year and seven days. Your estimated due date would be December 17, 2025.
This formula assumes a 28-day menstrual cycle with ovulation happening on day 14. If your cycles are consistently longer or shorter than 28 days, the estimate shifts. A person with a 35-day cycle ovulates roughly a week later than someone with a 28-day cycle, which would push the true due date about a week later than Naegele’s Rule suggests. If your cycles are irregular or you aren’t sure when your last period started, an early ultrasound gives a more reliable date.
Using Your Conception or Ovulation Date
If you know when you ovulated or conceived, you can skip the period-based math entirely. A full pregnancy lasts about 266 days from conception (compared to 280 days from the start of your last period, since those first two weeks before ovulation are counted even though you weren’t yet pregnant). Add 266 days to your conception date and you have your estimate.
This method is especially useful if you were tracking ovulation with test strips, basal body temperature, or fertility monitoring. It removes the guesswork about cycle length because you’re starting from the actual biological event rather than working backward from a period date.
How Ultrasound Dating Works
An early ultrasound is the most accurate tool for pinning down a due date. During the first trimester, a technician measures the baby from head to tailbone (called the crown-rump length). Before 14 weeks, this measurement predicts gestational age within 5 to 7 days. Before 9 weeks, the accuracy tightens to within about 5 days.
As pregnancy progresses, ultrasound dating becomes less precise. Babies grow at nearly identical rates in the first trimester, but by the second and third trimesters they start to vary in size based on genetics, nutrition, and other factors. That’s why early scans are the gold standard for setting a due date.
When an early ultrasound disagrees with a period-based calculation by more than 7 days, clinical guidelines recommend changing the due date to match the ultrasound. Before 9 weeks, even a 5-day gap is enough reason to adjust. This matters because an accurate due date affects decisions later in pregnancy, from scheduling routine tests to knowing when a pregnancy is considered overdue.
Due Dates After IVF or Embryo Transfer
If you conceived through IVF or another form of assisted reproduction, your due date calculation is actually more precise than for a natural pregnancy because you know the exact date the embryo was created and transferred.
The formula works like this: subtract the embryo’s age in days from the transfer date to get the conception date, then add 266 days. So if you had a day-5 embryo transferred on June 20, the conception date would be June 15 (five days earlier), and the due date would be 266 days from June 15. Your fertility clinic typically calculates this for you, but it helps to understand where the number comes from.
Why Your Due Date Might Change
It’s common for a due date to shift after that first ultrasound, and it doesn’t mean anything is wrong. The most frequent reason is that ovulation happened earlier or later than the standard day-14 assumption. Women with longer cycles, irregular periods, or uncertain period dates are especially likely to see an adjustment.
Your care provider will generally use whichever dating method is considered most reliable for your situation. If you had a clear, regular cycle and your ultrasound lines up within a few days, the original LMP-based date stands. If there’s a significant gap, the ultrasound wins. Once a due date is established in the first trimester, later ultrasounds shouldn’t be used to change it, since size differences in the second and third trimesters reflect the baby’s individual growth pattern rather than a dating error.
What “Due Date” Really Means
A due date marks the midpoint of a window, not a precise arrival time. Full-term birth spans from 39 weeks through 40 weeks and 6 days, and most babies arrive somewhere in the two-week range on either side of the estimated date. Only about 1 in 20 babies are born on the date itself. Thinking of it as a “due month” is closer to reality and can save you a lot of anxious waiting as that 40-week mark approaches.