A pregnancy due date is calculated by counting 280 days (40 weeks) from the first day of your last menstrual period. That’s the standard formula, and it’s the starting point for nearly every due date calculator, app, and clinical estimate. Only about 5% of babies actually arrive on that exact date, so think of it as the center of a window rather than a deadline.
The Standard 280-Day Formula
The most widely used method is called Naegele’s Rule. It works like this: take the first day of your last menstrual period (LMP), add seven days, then count back three months. If your last period started on March 10, you’d add seven days to get March 17, then go back three months to arrive at a due date of December 17. The math assumes a 28-day menstrual cycle and that ovulation happened on day 14 of that cycle.
This means the “40 weeks of pregnancy” includes roughly two weeks before you actually conceived. Fertilization typically happens about 266 days before delivery, but since most people know when their last period started and don’t know exactly when they ovulated, the LMP date is the practical starting point.
How Irregular Cycles Change the Math
The 280-day formula works best when your cycle runs close to 28 days. If your cycle is consistently longer, you likely ovulated later, which pushes your true due date further out. A shorter cycle means earlier ovulation and a slightly earlier due date. For example, if your cycle is regularly 35 days, you probably ovulated around day 21 instead of day 14, adding roughly a week to the standard calculation.
If your cycles are irregular, jumping from 21 days one month to 35 the next, an LMP-based estimate becomes unreliable. In these cases, an early ultrasound is the better way to pin down how far along you are. The same is true if you simply don’t remember when your last period started or if you became pregnant shortly after stopping hormonal birth control, which can delay the return of regular cycles.
Why Ultrasound Dating Is More Accurate
A first-trimester ultrasound (up to about 14 weeks) is the most accurate method for confirming gestational age. The technician measures the embryo from head to rump, and that measurement correlates tightly with how many weeks have passed. According to guidelines from the American College of Obstetricians and Gynecologists, this method is accurate to within 5 to 7 days.
That precision drops as pregnancy progresses. Babies grow at increasingly individual rates in the second and third trimesters, so an ultrasound at 30 weeks can’t pin down a due date as reliably as one at 8 weeks. This is why your provider may adjust your due date after an early ultrasound if it disagrees with the LMP calculation by more than a few days. Once an ultrasound-based due date is established early in pregnancy, it generally isn’t changed again later.
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. The formula subtracts the embryo’s age from the transfer date to find the equivalent conception date, then adds 266 days. For a Day 5 embryo transfer on June 1, the conception equivalent would be May 27 (five days earlier), and the due date would be 266 days from May 27. This removes the guesswork about ovulation timing entirely.
Paper Wheels vs. Digital Calculators
Many clinics still have cardboard pregnancy wheels sitting on countertops, but their accuracy is surprisingly uneven. A study that tested dozens of paper wheels found that only 35% produced a due date consistent with the standard 280-day pregnancy. The largest error was four days off, and two wheels from different manufacturers gave dates seven days apart from each other for the same LMP. None of the paper wheels corrected for leap years. Digital calculators and apps, by contrast, all produced the correct date and handled leap years automatically.
If your provider uses a paper wheel, it’s worth double-checking the result with a digital tool or a simple calendar count. A few days’ discrepancy rarely matters clinically, but it can affect decisions later in pregnancy about whether you’re considered early or overdue.
What “Due Date” Actually Means
A due date is an estimate, not a prediction. Full-term pregnancy spans from 39 weeks through 40 weeks and 6 days, and most babies arrive somewhere in a two-week window around the due date. Going a week past isn’t unusual. Going two weeks past typically prompts a conversation with your provider about next steps.
The date matters most as a reference point. It’s what your provider uses to schedule screenings at the right gestational age, interpret growth measurements, and decide when intervention might be appropriate if labor doesn’t start on its own. Knowing how it’s calculated helps you understand why your provider might shift it after an early ultrasound, or why the date from an app doesn’t quite match the one in your chart.