To calculate your due date from your last period, count 280 days (40 weeks) forward from the first day of your most recent menstrual period. That single date is the standard estimate used by nearly every pregnancy calculator, app, and prenatal chart. There’s also a simple shortcut you can do in your head without counting individual days.
The Three-Step Formula
The most widely used method is called Naegele’s Rule, and it works like this:
- 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.
So if the first day of your last period was 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 and that you ovulated on day 14 of that cycle. It’s the same math behind every online due date calculator. The 280-day count and Naegele’s Rule produce the same result; they’re just two ways of doing the same arithmetic.
Why the Count Starts Before Conception
One detail that confuses a lot of people: your pregnancy “clock” starts about two weeks before you actually conceived. Gestational age is measured from the first day of your last period, not from fertilization. Since ovulation typically happens around two weeks into a cycle, gestational age is roughly fertilization age plus two weeks.
This means that during the first two weeks of your 40-week pregnancy, you weren’t pregnant yet. It sounds odd, but it’s the standard because most people can pinpoint when their period started far more easily than when they ovulated. The entire system of prenatal care, from scheduling ultrasounds to tracking fetal growth, is built around this convention.
Adjusting for Longer or Shorter Cycles
Naegele’s Rule works cleanly for a 28-day cycle. If your cycles are consistently longer or shorter, you can adjust the estimate. The key is figuring out when you likely ovulated, since that’s the event that actually matters.
In a 28-day cycle, ovulation falls around day 14. If your cycle is 35 days, ovulation probably happened around day 21, which is seven days later than the formula assumes. In that case, you’d push your due date forward by seven days. If your cycle is 24 days, ovulation likely happened around day 10, four days earlier than expected, so you’d pull your due date back by four days.
The general rule: subtract 14 from your typical cycle length to estimate your ovulation day, then compare that to day 14. The difference is how many days to add or subtract from the standard 280-day estimate. If your cycles are irregular and you can’t identify a pattern, this adjustment won’t be reliable, and an early ultrasound will give you a better estimate.
How Accurate Is an LMP-Based Due Date?
Not as precise as most people assume. The 280-day calculation gives you a target, but it’s really the center of a wide window. Variability in cycle length and ovulation timing means the LMP date can easily be off by a week or more, even for someone with fairly regular periods. Factors like stress, travel, or recent use of hormonal birth control can shift ovulation without you realizing it.
The numbers bear this out. About half of first-time mothers give birth by 40 weeks and 5 days, with the other half delivering after that point. For mothers who have given birth before, the midpoint is 40 weeks and 3 days. Very few babies arrive on the exact calculated date.
Experts now define full-term birth as a five-week range rather than a single day:
- Early term: 37 weeks 0 days through 38 weeks 6 days
- Full term: 39 weeks 0 days through 40 weeks 6 days
- Late term: 41 weeks 0 days through 41 weeks 6 days
- Post term: 42 weeks 0 days or later
Think of your due date less as a deadline and more as the middle of a window that stretches roughly three weeks in either direction.
When Ultrasound Replaces the LMP Date
A first-trimester ultrasound (up to about 14 weeks) is the most accurate method for confirming or adjusting a due date. It measures the embryo or fetus directly rather than relying on assumptions about your cycle.
The standard clinical guideline is straightforward: if an ultrasound done before 14 weeks disagrees with your LMP-based date by more than 7 days, the due date gets changed to match the ultrasound. Smaller discrepancies may also prompt a change depending on how early the ultrasound was performed and how confident you are about your LMP date. An ultrasound at 7 or 8 weeks, for instance, is more precise than one at 13 weeks, so even a modest difference at that early stage could be enough to adjust the estimate.
If your pregnancy resulted from fertility treatments like IVF, the date is calculated from the known timing of the procedure rather than from your LMP, since that information is more precise than either a period date or an ultrasound.
First Pregnancies vs. Subsequent Ones
First-time mothers tend to carry slightly longer than those who have given birth before. The difference isn’t dramatic, about two days on average, but it’s consistent across large studies. If this is your first pregnancy and you go a few days past your due date, that’s well within the statistical norm. The 40-week estimate was never designed to be exact for any individual pregnancy; it’s an average across a large population with wide natural variation.
A Quick Example
Say the first day of your last period was June 1, 2025. Using Naegele’s Rule: count back three months to March 1, then add one year and seven days. Your estimated due date is March 8, 2026. Using the 280-day method, you’d count 280 days forward from June 1 and land on the same date.
If your cycles typically run 32 days instead of 28, you’d add four extra days, shifting the estimate to March 12, 2026. Either way, your first prenatal ultrasound will either confirm or fine-tune that number.