The standard method for calculating a delivery date starts from the first day of your last menstrual period (LMP) and counts forward 280 days, or 40 weeks. Only about 4% of babies arrive on their predicted due date, so think of it as an estimate rather than an appointment. Most babies are born somewhere between 37 and 42 weeks, a five-week window that counts as full term.
The Three-Step Formula
The most widely used calculation is called Naegele’s Rule, and you can do it without any tools. Johns Hopkins Medicine breaks it into three steps:
- Step 1: Find the first day of your last menstrual period.
- 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, 2024, then add one year and seven days. Your estimated due date would be December 17, 2025.
This formula assumes a 28-day menstrual cycle. If your cycle is consistently longer or shorter than that, the estimate needs adjusting.
Adjusting for Irregular or Longer Cycles
Naegele’s Rule works best when your cycle runs like clockwork at 28 days. For cycles that don’t fit that pattern, a simple correction known as Wood’s Rule shifts the date forward or backward based on your actual cycle length.
If your cycle is longer than 28 days, add the extra days to the due date you got from Naegele’s Rule. If your cycle is shorter, subtract the difference. So if your cycle typically runs 35 days, you’d add 7 days to the standard estimate. If it runs 25 days, you’d subtract 3 days. The logic is straightforward: a longer cycle means you likely ovulated later than day 14, pushing conception (and therefore delivery) further out.
This adjustment matters more than many people realize. A woman with a 35-day cycle who uses the standard formula without correcting could end up with a due date that’s a full week too early, which can lead to unnecessary worry about being “overdue” or even premature induction discussions.
Why the Count Starts Before Conception
One detail confuses a lot of people: the 40-week countdown begins about two weeks before the baby was actually conceived. Gestational age counts from the first day of your last period, but ovulation and fertilization typically happen around two weeks later. That means when you’re told you’re 10 weeks pregnant, the embryo is closer to 8 weeks old.
This isn’t a mistake. Doctors use gestational age because most people can identify when their last period started far more reliably than when they ovulated. It’s a practical convention, not a biological one. But it does mean the “40 weeks of pregnancy” includes roughly two weeks when you weren’t actually pregnant yet.
How Ultrasound Refines the Estimate
A first-trimester ultrasound, typically done between 8 and 12 weeks, is the most accurate way to confirm or adjust a due date. The technician measures the embryo from crown to rump, and because early growth follows a very predictable pattern across all pregnancies, this measurement can pin down gestational age to within a few days.
If the ultrasound date and the LMP-based date disagree by more than about a week, your provider will usually go with the ultrasound. Later ultrasounds are less reliable for dating because babies start growing at different rates in the second and third trimesters. A scan at 30 weeks might be off by two to three weeks, which is why early dating scans carry so much more weight.
Calculating a Due Date After IVF
If you conceived through IVF, the calculation is more precise because you know the exact date of embryo transfer. Rather than estimating backward from a period, the formula counts forward from the transfer date. For a day-5 embryo transfer (the most common type with frozen embryos), you add 261 days, or equivalently treat the transfer date as day 19 of a standard cycle and count 40 weeks from the corresponding “LMP.”
Whether the embryo was frozen on day 5, day 6, or day 7, the due date calculation stays the same, since all blastocyst-stage transfers are treated identically for dating purposes. The date of egg retrieval can also serve as the starting point, with an appropriate adjustment for the days of lab culture before transfer.
How Accurate the Estimate Really Is
A due date is a midpoint of a probability curve, not a prediction. Only 4% of babies are born on the exact date, and that figure barely budges (to 4.4%) even when you exclude premature births and complicated pregnancies. The practical reality is that your baby will most likely arrive sometime in a window stretching from 37 weeks to 42 weeks.
Several factors influence where in that window you’ll fall. Research from the University of Chicago found that maternal age, number of previous births, pre-pregnancy weight, and even coffee and alcohol use during pregnancy all play a role in actual pregnancy length. First-time mothers tend to carry longer than women who have given birth before. These variables are why two women with the same LMP can deliver weeks apart, and why a more personalized model that accounts for individual factors outperforms the simple 280-day rule.
None of this means due dates are useless. They give your care team a reference point for tracking fetal growth, scheduling tests, and identifying pregnancies that run unusually long or short. Just don’t circle the date on your calendar in permanent marker. Packing your hospital bag a few weeks early is a better strategy than counting on precision.