How to Calculate Your Due Date (Pregnancy)
3 min read
A due date is an estimate of when a baby is likely to arrive - around 40 weeks from the start of the last menstrual period. Only a small share of babies arrive exactly on it, but it is a useful anchor for prenatal care.
Here is how the due date calculation works.
Step by step
- 1Find the first day of your last period (LMP)
The standard method counts from the first day of your last menstrual period, not from conception, because that date is easier to know.
- 2Apply Naegele's rule
Add one year to the LMP, subtract three months, then add seven days. For an LMP of March 1, that gives a due date around December 8.
- 3Or count 40 weeks
Equivalently, the due date is about 280 days (40 weeks) after the first day of the LMP.
- 4Adjust for cycle length or conception date
If you know your conception date, add 38 weeks (266 days) instead. The Due Date Calculator handles either method for you.
Tips
- A due date is an estimate - a normal full-term birth can happen anytime from 37 to 42 weeks.
- An early ultrasound often gives a more accurate date than the LMP, especially with irregular cycles.
- Always confirm your dates and any concerns with your doctor or midwife.
Frequently asked questions
How is a due date calculated?
Most commonly by Naegele's rule: take the first day of your last period, add a year, subtract three months, and add seven days - about 40 weeks from your last period.
Is the due date exact?
No. It is an estimate of the most likely window. Births are considered full term anywhere from 37 to 42 weeks, and only a few arrive on the exact date.
Can I calculate it from conception instead?
Yes. From a known conception date, add about 38 weeks (266 days). The calculator supports both the last-period and conception methods.