Naegele's rule
The most common way to estimate a due date is Naegele's rule: take the first day of your last menstrual period (LMP), add one year, subtract three months and add seven days. That's the same as adding 280 days, or 40 weeks. If your last period began on March 1, your estimated due date is around December 6. Our pregnancy due date calculator applies this automatically.
Why 40 weeks?
Pregnancy is counted from the start of the last period, roughly two weeks before conception in a typical 28-day cycle. That's why a pregnancy is described as 40 weeks long even though the baby develops for about 38 weeks.
Longer or shorter cycles
If your cycles are longer than 28 days, ovulation usually happens later, so the due date shifts later as well; shorter cycles shift it earlier. Many calculators let you adjust for cycle length.
Ultrasound dating
An early ultrasound, especially in the first trimester, measures the embryo and is often more accurate than the LMP method. If the two estimates differ significantly, healthcare providers usually go with the ultrasound date. For IVF pregnancies, the transfer date gives a very precise estimate.
Only a few babies arrive on the due date
Only around 4 to 5% of babies are born on their exact due date. A full-term pregnancy is considered 39 to 40 weeks and 6 days, and most births happen within about two weeks either side of the estimate. Think of the due date as the middle of a window.
Trimesters
The first trimester runs from week 1 to the end of week 13, the second from week 14 to week 27, and the third from week 28 until birth. Each brings different checkups and milestones.
Counting weeks and days
To see how many weeks pregnant you're, count the days from your LMP and divide by 7. Our days between dates calculator is useful for this, and for counting down to the due date.
Important note
This information is general. Always follow the dates and advice given by your doctor or midwife, who can account for your personal history.
Frequently asked questions
How is the due date calculated?
Add 280 days (40 weeks) to the first day of your last menstrual period.
Is ultrasound more accurate than the LMP method?
Early first-trimester ultrasound is generally considered more accurate.
How many babies are born on their due date?
Only about 4–5%.
When does the third trimester start?
At week 28.