How the due date is calculated
This calculator uses Naegele's rule, the method most healthcare providers start with: your estimated due date is 280 days (40 weeks) after the first day of your last menstrual period. The rule assumes a 28-day cycle with ovulation around day 14. If your cycle is shorter or longer, the date is shifted by the difference, which is why this tool asks for your average cycle length.
Gestational age explained
Pregnancy is counted from the first day of your last period, not from conception, so you are already "two weeks pregnant" at the time of ovulation. That is why a pregnancy lasts about 40 weeks even though the baby develops for roughly 38 weeks after conception.
How accurate is a due date?
Only around 4 to 5 percent of babies are born on their exact due date. Most are born within a couple of weeks either side. Providers often confirm or adjust the date using an ultrasound in the first trimester, which can be more accurate than the last period if your cycles are irregular or you are unsure of the date.
The three trimesters
- First trimester: up to the end of week 13.
- Second trimester: weeks 14 to 27.
- Third trimester: week 28 until birth.
Frequently asked questions
How do I calculate my due date?
Add 280 days (40 weeks) to the first day of your last menstrual period. If your cycle is not 28 days, add or subtract the difference.
What if I do not remember my last period?
Your healthcare provider can estimate your due date with an ultrasound, which measures the size of the baby early in pregnancy.
When is a baby considered full term?
Full term is 39 weeks through 40 weeks and 6 days. Babies born earlier are called early term or preterm depending on how early.
Is this a medical diagnosis?
No. It is an estimate for planning only. Always follow the dating and care advice of your doctor or midwife.