A pregnancy due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period. This is the standard method, and it assumes a 28-day cycle with ovulation on day 14. If you know your conception date instead, add 266 days. Either way, the result is an estimate, not a guarantee.
The Standard Formula: Naegele’s Rule
The most widely used method is called Naegele’s Rule. Here’s how it works in three steps:
- Step 1: Find the first day of your last menstrual period (LMP).
- Step 2: Count back 3 calendar months from that date.
- Step 3: Add 1 year and 7 days.
So 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. It also assumes that conception happened around day 14 of that cycle, which is when ovulation typically occurs. If your cycle is consistently longer or shorter than 28 days, the estimate shifts. A 35-day cycle, for example, means you likely ovulated around day 21 instead of day 14, pushing your true due date about a week later than what the standard formula gives you. The simplest adjustment: add or subtract the difference between your cycle length and 28 days.
Calculating From a Known Conception Date
If you know exactly when you conceived (or very close to it), the math is simpler: add 266 days to your conception date. This skips the two-week buffer that Naegele’s Rule builds in to account for the time between the start of your period and ovulation. The 280-day and 266-day methods land on the same date when ovulation happens on cycle day 14. They only diverge when it doesn’t.
Knowing your conception date is uncommon outside of fertility treatment, though. Most people who track ovulation through temperature charting or ovulation kits can narrow it down to a window of a few days, which still makes this a useful approach.
How IVF Due Dates Are Calculated
For pregnancies conceived through IVF, the calculation starts from the embryo transfer date. The formula is: conception date equals the transfer date minus the embryo’s age in days, and then you add 266 days to get the due date.
If you had a day-5 blastocyst transfer on January 20, your conception date would be January 15 (five days earlier). Add 266 days, and you have your estimated due date. Day-5, day-6, and day-7 frozen embryo transfers all use the same calculation because they’re all considered blastocyst-stage transfers. Day-3 transfers simply subtract 3 instead of 5. Because IVF pinpoints the exact timing of conception, these due dates tend to be more precise than period-based calculations.
Why Ultrasound Often Changes Your Due Date
A first-trimester ultrasound is the most accurate tool for confirming or adjusting a due date. Before 14 weeks, the measurement from the top of the baby’s head to the bottom of their body (called crown-rump length) can estimate gestational age within 5 to 7 days. That’s a tighter window than most period-based calculations can offer, especially for anyone with irregular cycles or uncertain period dates.
In one study, 40% of women who received a first-trimester ultrasound had their due date changed because the ultrasound disagreed with their period-based date by more than 5 days. Current guidelines say that before 9 weeks, a discrepancy of more than 5 days between the ultrasound and the period-based date is enough reason to update the due date. The earlier the ultrasound, the more reliable it is, because embryos grow at very predictable rates in the first trimester. Later in pregnancy, natural size variation between babies makes ultrasound dating less precise.
How Accurate the Standard Formula Really Is
Naegele’s Rule has been the default for over 200 years, but it wasn’t built on data. It’s based on the assumption that human pregnancy lasts exactly 10 lunar cycles (nine months plus seven days), and it doesn’t account for any individual variation.
Researchers at the University of Chicago developed an alternative model that factors in 16 variables, including maternal age, pre-pregnancy weight, race, education, and coffee and alcohol use. A study comparing this model to Naegele’s Rule found it was twice as accurate at predicting the actual delivery date. That model isn’t widely used in everyday practice, but its existence highlights an important point: the standard formula is a rough average, not a personalized prediction.
Only about 5% of babies arrive on their due date. Most full-term deliveries happen within a two-week window on either side. Your due date is best understood as the midpoint of a range, not a target.
Factors That Shift Your Due Date
Several things can make a due date less reliable or push the actual delivery earlier or later:
- Irregular cycles: If your periods don’t follow a predictable pattern, your ovulation timing is harder to pin down, and a period-based due date is less trustworthy. Ultrasound dating becomes especially important.
- Longer or shorter cycles: A cycle that’s consistently 35 days rather than 28 means ovulation happens later. The standard formula would set your due date about a week too early unless you adjust for it.
- Uncertain period dates: If you’re not sure when your last period started, or if you had spotting that could be confused with a period, the LMP-based calculation loses accuracy.
- First pregnancy vs. subsequent pregnancies: First babies tend to arrive slightly later on average. Some adjusted models account for this, though the standard formula does not.
When there’s any uncertainty, an early ultrasound is the most reliable way to establish dating. If your provider changes your due date after an ultrasound, it’s because the measurement data is more precise than the calendar-based estimate.