How Is Your Due Date Calculated?
Your due date shapes the whole pregnancy timeline — but where does that single date actually come from? This guide explains the standard method, why it starts before conception, and how accurate the estimate really is.
The standard due date is 280 days (40 weeks) from the first day of your last period (Naegele's rule): add a year, subtract three months, add seven days. Because conception is ~2 weeks after that, it's really ~38 weeks from conception. Only about 1 in 20 babies arrives on the exact date.
The Short Answer
The standard due date is calculated as 280 days (40 weeks) from the first day of your last menstrual period (LMP) — a method called Naegele's rule. A quick version: take your LMP, add one year, subtract three months, then add seven days. Only about 1 in 20 babies actually arrives on the exact due date.
How the Due Date Is Calculated
Naegele's rule counts 40 weeks from the first day of your last period. For example, if your LMP began on January 1, add a year (Jan 1 next year), subtract three months (October 1), and add seven days — an estimated due date of about October 8. Pregnancy is counted in 40 weeks split into three trimesters.
Why It's Measured From Your Last Period
It may seem odd to start counting before conception even happens, but LMP is the most reliable date most people can pinpoint. Conception usually occurs about two weeks after the LMP, so a "40-week" pregnancy is really about 38 weeks from conception. That's also why you're considered "4 weeks pregnant" around the time of a missed period.
The 40-Week Timeline
| Trimester | Weeks (from LMP) |
|---|---|
| First trimester | Weeks 1–13 |
| Second trimester | Weeks 14–27 |
| Third trimester | Weeks 28–40 |
Ultrasound Dating Is More Precise
If your cycles are irregular or you're unsure of your LMP, an early ultrasound gives a more accurate date. First-trimester ultrasounds (before about 14 weeks) estimate gestational age within a few days by measuring the baby. Doctors often adjust the LMP-based due date if an early scan differs significantly.
What Can Shift Your Due Date
Irregular or long cycles, uncertainty about your LMP, and early ultrasound measurements can all move the estimate. Your provider uses the most reliable information available — often the early scan — to set the date they'll track your pregnancy by.
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Frequently Asked Questions
How is my due date calculated?
The standard method (Naegele's rule) adds 280 days — 40 weeks — to the first day of your last menstrual period. A shortcut is to take your last period's start date, add one year, subtract three months, and add seven days.
Why is pregnancy counted from the last period?
Because the last menstrual period is the most reliable date to pinpoint. Conception happens about two weeks later, so a 40-week pregnancy measured from your last period is really about 38 weeks from conception.
How accurate is a due date?
It's an estimate. Only about 4–5% of babies are born on the exact due date, though most arrive within roughly two weeks on either side. Full term is considered 39–40 weeks, with normal delivery from 37 to 42 weeks.
Is an ultrasound or LMP more accurate for due dates?
An early (first-trimester) ultrasound is generally more accurate, especially if your cycles are irregular or you're unsure of your last period. Providers often adjust the due date if an early scan differs from the LMP estimate.
Can my due date change?
Yes. It can be adjusted based on early ultrasound measurements, irregular cycles, or uncertainty about your last period. Your provider sets the date using the most reliable information available.