Due Date Calculator: How Pregnancy Due Date Is Calculated (Week-by-Week Guide)
The formula behind every estimated due date, a week-by-week trimester breakdown, and why the date is a statistical estimate, not a deadline.
Due date = first day of your last period + 280 days (40 weeks), adjusted for your actual cycle length if it's not 28 days. This is Naegele's rule, the standard method used by essentially every due-date calculator and healthcare provider.
Get your exact estimated date, current week, and trimester in our pregnancy due date calculator.
Almost every due date you'll ever see — from an app, a calculator, or a doctor's initial estimate — comes from the same formula: Naegele's rule. It's simple, it's been the standard for well over a century, and it starts counting from a date that isn't actually conception.
The Formula (Naegele's Rule)
280 days assumes ovulation on day 14 of a standard 28-day cycle. If your average cycle runs longer or shorter, ovulation shifts accordingly — which is why the adjustment matters for anyone whose cycle isn't textbook-standard.
Why Count From the Last Period, Not Conception?
Conception is hard to pinpoint exactly — most people don't know the precise day it happened. The first day of the last period, by contrast, is a specific, rememberable date. That's why medicine standardized on it as the starting point, even though it means "6 weeks pregnant" actually means about 4 weeks since conception — a detail that trips a lot of people up the first time they see it.
Week-by-Week: The Trimesters
| Trimester | Weeks | Roughly |
|---|---|---|
| First | 1-12 | Months 1-3 |
| Second | 13-26 | Months 4-6 |
| Third | 27-40+ | Months 7-9+ |
Because weeks 1-2 are counted before conception even occurs, "week 4" is typically around the time a pregnancy is first detectable, and the first trimester is already a third over by the time most people find out.
Worth knowing: Only about 5% of babies are born on their exact estimated due date. Most arrive within roughly two weeks of it, before or after — treat the date as a planning midpoint, not a deadline to circle on the calendar.
Worked Example
Last period started on a given date, with a standard 28-day cycle: adding 280 days lands the due date almost exactly 9 months and 1 week later. For someone with a 32-day cycle (4 days longer than standard), the adjustment shifts ovulation — and the due date — about 4 days later than a straight 280-day count would suggest.
When an Ultrasound Estimate Differs
An early ultrasound measures the size of the developing embryo or fetus directly, which is often more accurate than a last-period-based estimate — especially for anyone with an irregular cycle, where the 28-day assumption breaks down. It's common and normal for a healthcare provider to adjust the initially calculated due date after an ultrasound.
IVF Due Dates — A Different Starting Point
For pregnancies achieved through IVF, the due date isn't calculated from the last menstrual period at all, since the exact date of fertilization and transfer is already known precisely — Naegele's rule's two-week uncertainty simply doesn't apply the same way.
| Transfer type | Due date formula |
|---|---|
| Day 3 embryo transfer | Transfer date + 263 days (280 − 17 days for a day-3 embryo's developmental age) |
| Day 5 embryo transfer (blastocyst) | Transfer date + 261 days (280 − 19 days for a day-5 embryo's developmental age) |
Because the exact embryo age at transfer is known, IVF due dates are generally considered more precise than a standard last-period-based estimate for a naturally conceived pregnancy.
How Due Dates Get Revised During Pregnancy
It's common and normal for an estimated due date to be adjusted at least once. An early first-trimester ultrasound (typically weeks 8-13) measures the embryo or fetus directly and is generally considered the most accurate dating method overall — if it disagrees with the last-period-based estimate by more than about a week, healthcare providers usually update the due date to match the ultrasound rather than the original calculation. Later ultrasounds are used less for dating (growth becomes more individual-specific as pregnancy progresses) and more for monitoring development against the already-established date.
What "Full Term" Actually Means
Not every week close to the due date carries the same label — the terminology itself reflects that a due date is a range, not a single target:
| Term | Week range |
|---|---|
| Early term | 37 weeks 0 days – 38 weeks 6 days |
| Full term | 39 weeks 0 days – 40 weeks 6 days |
| Late term | 41 weeks 0 days – 41 weeks 6 days |
| Post-term | 42 weeks 0 days and beyond |
This classification exists precisely because "the due date" was never meant to be read as a single deadline — a birth anywhere across the early-to-late-term range is considered within normal timing.
Frequently Asked Questions
How is a pregnancy due date calculated?
Using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period, adjusted for a cycle length other than the standard 28 days.
Why is pregnancy dated from the last period instead of conception?
Conception date is usually unknown or hard to pinpoint exactly, while the first day of the last period is a specific, rememberable date — so it became the medical standard starting point, even though conception happens roughly two weeks later.
How accurate is the estimated due date?
Only about 5% of babies are born on their exact estimated due date. Most arrive within roughly two weeks of it, before or after — it's a statistical midpoint, not a deadline.
What are the three trimesters by week?
First trimester: weeks 1-12. Second trimester: weeks 13-26. Third trimester: weeks 27 through birth, typically around week 40.
Is an ultrasound more accurate than the last-period calculation?
Yes, generally — an early ultrasound measurement is considered more accurate than a last-period-based estimate, especially for people with irregular cycles, and healthcare providers often use it to confirm or adjust the due date.
