Pregnancy Due Date Calculator
A due date is the midpoint of a range, not a deadline. Fewer than one baby in twenty arrives on it.
Naegele's rule
The standard estimate, in use since Franz Naegele formalised it in the early nineteenth century, adds 280 days to the first day of the last menstrual period:
Note what it counts from. Gestational age is measured from the last period, not from conception, which means you are considered two weeks pregnant at the moment of conception. This is a convention, not an error: the last period is a date most people can identify, whereas conception usually is not.
The rule assumes a 28-day cycle with ovulation on day 14. If your cycles run longer or shorter, this calculator shifts the estimate accordingly — a 35-day cycle moves the due date a week later.
How accurate is a due date
Only about 4 % of babies arrive on their estimated due date. Roughly 60 % are born within a week either side, and around 90 % within two weeks. The clinically normal window for a full-term birth spans 37 to 42 weeks — a five-week range.
An early ultrasound is more accurate than the date calculation. A crown-rump length measurement between 8 and 13 weeks predicts delivery within about ±5 days, and most maternity services will redate a pregnancy if the scan differs from the menstrual estimate by more than about a week. If your dates and your scan disagree, the scan generally wins.
What the milestones mean
| Week | Significance |
|---|---|
| 13 | End of the first trimester; miscarriage risk drops sharply |
| 20 | Anomaly scan in most health systems |
| 24 | Threshold of viability with intensive neonatal care |
| 28 | Start of the third trimester |
| 37 | Early term — no longer considered premature |
| 39–40 | Full term; best outcomes on average |
| 42 | Post-term; induction usually discussed before this |
This is an estimate, not maternity care. It cannot account for irregular cycles, IVF timing, a scan-adjusted date, multiple pregnancy or any clinical factor. Your midwife or doctor's date supersedes anything calculated here, and any concern about your pregnancy should go to them rather than to a website.
Frequently asked questions
Why does pregnancy count from my last period?
Because it is the one date most people can pin down. Conception typically occurs around two weeks later, which is why the 40-week count corresponds to about 38 weeks of actual fetal development. Every clinical guideline uses this convention, so a 'weeks pregnant' figure always means gestational age from the last period.
What if my cycles are irregular?
Then the menstrual estimate is unreliable, because ovulation timing is the variable the rule assumes away. An early dating scan is much the better option, and is standard practice in that situation.
Does this work for IVF pregnancies?
No. With IVF the transfer date is known precisely, so the due date is calculated from it: 266 days from a day-5 blastocyst transfer minus 5 days, or 266 days from a day-3 transfer minus 3 days. Your clinic will give you the exact date.