To calculate your estimated due date (EDD), take the first day of your last menstrual period, count back three calendar months, then add one year and seven days. This gives you a date roughly 280 days, or 40 weeks, from the start of your last period. It’s the most widely used method, but it’s not the only one, and several factors can shift the estimate.
Naegele’s Rule: The Standard Method
The classic formula used by most providers is called Naegele’s Rule, and it works in three steps:
- Step 1: Identify 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.
So if your last period started on March 10, 2025, you’d count back three months to December 10, 2024, then add one year and seven days to get December 17, 2025. The math assumes a 28-day menstrual cycle and that you ovulated on day 14. If your cycles are longer or shorter than 28 days, this estimate may be off by several days or more.
If You Know the Conception Date
When you know exactly when conception occurred, the calculation is simpler: add 266 days (38 weeks) to that date. This is useful when ovulation was tracked with testing kits or temperature charting. The reason it’s 266 days instead of 280 is that Naegele’s Rule includes roughly two weeks before ovulation even happens. When you start counting from conception itself, those two weeks drop off.
Adjusting for Irregular Cycles
Naegele’s Rule can miss the mark if your cycles don’t run on a 28-day schedule. A formula developed by Dr. Parikh accounts for this by swapping out the standard seven-day addition with a personalized number. You take the average length of your previous cycles, subtract 21, and use that number in place of the seven days in Naegele’s Rule.
For example, if your typical cycle is 35 days, you’d subtract 21 to get 14. Instead of adding seven days in the final step, you’d add 14 days. If your cycle runs short at 24 days, you’d add only three days. This adjustment can shift your due date by a week or more compared to the standard calculation, which matters when it comes to decisions later in pregnancy about whether you’re overdue.
How Ultrasound Dating Works
An early ultrasound is the most accurate tool for confirming or correcting a due date. During the first trimester, your provider measures the embryo from head to tailbone (called crown-rump length) and compares it against growth charts to estimate gestational age. In the first trimester, this measurement is accurate to within about three days.
The American College of Obstetricians and Gynecologists has specific guidelines for when an ultrasound should override a period-based due date. Before nine weeks, a discrepancy of more than five days is enough reason to change the EDD. Between nine and fourteen weeks, the threshold is seven days. If you’re unsure of when your last period started, your provider will rely on the earliest available ultrasound to set the date.
Ultrasound dating becomes less precise as pregnancy progresses because babies start growing at different rates. This is why that first-trimester scan carries so much weight.
Calculating After IVF or Fertility Treatment
If pregnancy resulted from IVF or another assisted reproduction method, the due date calculation uses the embryo transfer date. You subtract the age of the embryo at the time of transfer to find the equivalent conception date, then add 266 days. For a five-day embryo transferred on June 1, the conception date would be May 27, and you’d count 266 days forward from there.
Clinical guidelines recommend using this ART-derived date rather than ultrasound or LMP calculations, since the timing of conception is known precisely.
Twin and Multiple Pregnancies
The EDD calculation itself doesn’t change for twins, but the realistic delivery window does. A large study of nearly 89,000 twin births in Japan found a mean gestational age at delivery of 37 weeks, and the risk of stillbirth and neonatal complications rises after 38 weeks. So while the formula might give you a 40-week date, your provider will likely plan around a delivery window closer to 37 to 38 weeks.
What Affects Accuracy
Several biological factors influence how long a pregnancy actually lasts. Maternal age, whether this is a first pregnancy, and race all play a role. First-time mothers tend to carry longer than those who’ve given birth before. Women younger than 19 or older than 34 tend to have shorter pregnancies on average.
The bottom line on accuracy is humbling. About 68% of people give birth within 11 days of their ultrasound-estimated due date. That’s a 22-day window where birth is most likely, not a single target date. Your EDD is the center of a bell curve, not a deadline. Planning around a range of two to three weeks on either side gives you a more realistic picture of when your baby will arrive.