A due date can feel wonderfully specific — a single square on the calendar you start counting down to. In reality, it’s an educated estimate, and it’s helpful to understand where that estimate comes from. Knowing how the number is reached makes it easier to hold it loosely, which is exactly how your care team treats it too.
Naegele’s rule: the classic calculation
The traditional formula, known as Naegele’s rule, is beautifully simple: take the first day of your last menstrual period (LMP) and add 280 days — that’s 40 weeks. A common shortcut is to count back three months from the first day of your last period and add seven days and a year. Either way, you land on the same estimated due date.
This is the math behind most due date tools, including our due date calculator, which lets you start from either your last period or a known conception date.
A worked example
It’s easier to see the rule in action with real dates. Say the first day of your last period was March 10, 2026. To apply Naegele’s rule the long way, you’d add 280 days to that date, landing on December 15, 2026. The mental shortcut gets you to the same place: count back three months from March 10 to reach December 10, then add seven days to reach December 17 — and because you’ve crossed into a new calendar year, you tick the year forward. The two approaches differ by a day or two depending on month lengths and leap years, which is exactly why a calculator is handy; it counts the actual days rather than approximating with whole months. Either way, you can see the estimate takes shape from nothing more than one remembered date and some steady arithmetic.
Why it counts from your last period, not conception
Here’s a quirk that surprises many people: pregnancy is measured from the first day of your last period, which is roughly two weeks before you actually conceive. It sounds odd to be counted as “pregnant” before egg and sperm have met, but there’s a practical reason. Almost everyone can name the day their last period started; very few know the precise day they ovulated or conceived. Using the LMP gives providers a consistent, comparable starting line for every pregnancy.
If you’d rather think in terms of conception, our conception calculator works backward to estimate when it likely happened.
The role of cycle length
Naegele’s rule quietly assumes a textbook 28-day cycle with ovulation around day 14. Real cycles vary. If yours tends to run longer — say 32 or 35 days — you probably ovulate later, which can push your true due date a few days past what the standard formula predicts. Shorter cycles can nudge it the other way. This is one reason a calculated date is a starting point, not the final word, and why your provider may fine-tune it.
If you know your typical cycle length, you can account for some of this yourself. A rough approach is to adjust the standard estimate by the difference between your cycle and the textbook 28 days — a 32-day cycle suggests ovulation about four days later, which would move a naive LMP-based date roughly four days later too. Our conception calculator can help you reason about ovulation timing, but this remains an approximation. The most reliable correction still comes from an early scan, which measures the pregnancy directly rather than inferring dates from cycle patterns.
Dating methods compared
| Method | How it works | Accuracy |
|---|---|---|
| Naegele’s rule (LMP) | Adds 280 days to the first day of your last period | Good if cycles are regular; less reliable with irregular cycles or an uncertain LMP |
| Ultrasound dating | Measures the baby, especially first-trimester crown-rump length | Most accurate, particularly early in pregnancy |
| IVF dating | Counts from the known egg-retrieval or embryo-transfer date | Very precise, because the timing is documented |
| Conception date | Adds 266 days to a known or estimated date of conception | Only as accurate as your knowledge of when you conceived |
Ultrasound dating: the most accurate method
If there’s one takeaway here, it’s this: an early ultrasound is the most accurate way to date a pregnancy. In the first trimester, measuring the baby’s crown-rump length — the distance from head to bottom — gives a reliable estimate because babies grow at a very predictable rate at this stage. If your scan date differs from your calculated date by more than a few days, your provider will typically update your due date to match the ultrasound. Later scans can still estimate age, but they become less precise as babies begin to grow at more individual rates.
When your due date gets revised
Providers don’t swap your date for the scan’s on a whim — they follow sensible thresholds. Broadly, the earlier the ultrasound and the larger the gap between it and your LMP-based estimate, the more likely your date is updated. A scan in the first several weeks that disagrees with your period dates by more than about five to seven days will usually take over, because measurements are so consistent that early. As pregnancy progresses, the allowable discrepancy grows: by the second trimester it takes a difference of a couple of weeks before a change is made, since normal variation in growth widens. Once a due date is set by a good early scan, it generally isn’t moved again by later measurements — a baby measuring ahead or behind at 30 weeks is usually a note about growth, not a reason to re-date the pregnancy. Your provider will explain any revision and why they made it.
IVF dating
Pregnancies conceived through IVF come with an unusually precise starting point, because the date of egg retrieval or embryo transfer is documented to the day. Your clinic calculates the due date from that known milestone rather than from an estimated ovulation, which removes much of the guesswork built into the standard formula.
Why so few babies arrive on the exact date
It’s worth setting expectations gently: only about 1 in 20 babies is born on the exact due date. A full-term pregnancy spans a range rather than a single day, most often clustering around 39 to 40 weeks. Arriving in the days or couple of weeks on either side of your due date is completely normal. The date is best understood as the center of a window — a helpful anchor for planning, not a countdown to a guaranteed arrival.
Your due date is an estimate, not a deadline
It helps to know the language your care team uses for timing, because it reframes the whole idea of a “due” date. A pregnancy is considered early term from 37 weeks, full term from 39 through 40 weeks, late term at 41 weeks, and post-term once it passes 42 weeks. That’s a spread of more than a month in which a baby can arrive perfectly healthily. Your due date sits right in the middle of the full-term window, which is why so few arrivals land exactly on it and why the days around it feel like such a long stretch of waiting.
Understanding this can take real pressure off the final weeks. If your date comes and goes with no baby yet, it doesn’t mean anything has gone wrong — it means you’re simply moving through the normal range. Your provider will keep a close eye as you approach and pass your date, and will talk with you about monitoring or, if needed, options like induction as the pregnancy nears 41 to 42 weeks. The number on the calendar is a planning tool and a reassurance that things are progressing, not a promise your baby signed. Holding it loosely is not just easier on your nerves; it’s the medically honest way to see it.
The bottom line
Your due date blends simple arithmetic with, ideally, an early ultrasound to sharpen the estimate. Cycle length, conception timing and IVF records all feed into it. Use it to plan and prepare, lean on your first-trimester scan for the most reliable date, and remember that your baby will ultimately choose their own moment. If you ever have questions about how your date was set or updated, your provider is the best person to walk you through it.