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Due Date Calculator
Estimated due date, with how little the date means.
What it does. The standard estimate adds 280 days to the first day of your last period — Naegele's rule. It assumes a 28-day cycle with ovulation on day 14, so a longer or shorter cycle shifts the date. Only about 4% of babies are born on their estimated date.
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How to use Due Date Calculator
- Enter the first day of your last period, or a conception date.
- Adjust the cycle length if yours is not 28 days.
- Read the estimate, the likely window and trimester dates.
Why the count starts before conception
Pregnancy is dated from the first day of the last period, not from conception, because that is the date people can actually identify. Ovulation is typically about two weeks later.
So at the moment of conception you are already counted as two weeks pregnant, which is why the 40-week total covers roughly 38 weeks of actual gestation.
This is the single most confusing part of pregnancy dating, and it is the reason a "six weeks pregnant" result can arrive only about two weeks after conception.
Cycle length changes the date
Naegele's rule assumes ovulation on day 14. If your cycles run 32 days, ovulation is later, and a straight 280-day count puts the due date roughly four days early.
Adjusting for cycle length corrects this, which is why the option is here.
An early ultrasound is more accurate than any calculation. In the first trimester, crown-rump length dates a pregnancy within about five days, and clinicians will usually revise the date if the scan differs from the calculated one by more than about a week.
The due date is a midpoint, not a deadline
| Timing | Weeks | Term status |
|---|---|---|
| Preterm | Before 37 | Early |
| Early term | 37 – 38+6 | Term |
| Full term | 39 – 40+6 | Term |
| Late term | 41 – 41+6 | Term |
| Post term | 42 and beyond | Prolonged |
Roughly 4% of babies arrive on the estimated date, and most births fall within a two-week window either side. The date is the centre of a distribution, and it is worth saying that to anyone who asks when the baby is "due".
Frequently asked questions
How is a due date calculated?
Naegele's rule adds 280 days to the first day of your last period, assuming a 28-day cycle with ovulation on day 14. A longer or shorter cycle shifts the date, which this tool adjusts for.
Why does pregnancy start counting before conception?
Because the first day of your last period is a date you can identify, while conception usually is not. At conception you are already counted as two weeks pregnant.
How many babies are born on their due date?
About 4%. Most births fall within roughly two weeks either side — the due date is the centre of a distribution, not a deadline.
Is an ultrasound more accurate than this calculation?
Yes. First-trimester crown-rump length dates a pregnancy within about five days, and clinicians usually revise the date if the scan differs from the calculated one by more than about a week.
What if I do not know my last period date?
Enter a conception or transfer date instead if you know one. Otherwise an early ultrasound is the reliable route — a calculator cannot date a pregnancy without a starting point.