Pregnancy Due Date Calculator
Estimate your due date from your last period or conception date, with your current week and trimester.
The last-period method follows Naegele's rule and is the standard clinical starting point. Your provider may adjust the date after an ultrasound.
Enter a date to see your estimated due date.
How to use the due date calculator
Choose the method you have a date for. Most people start with the first day of their last period, since that is the figure clinics use first.
- Pick the last-period method or the conception-date method.
- Enter the date in the field.
- Read your estimated due date at the top of the result.
- If the pregnancy is in progress, see how many weeks and days along you are today, plus the trimester.
How the due date is calculated
The last-period method uses Naegele's rule: it counts 280 days, or 40 weeks, from the first day of your last menstrual period. Pregnancy is dated this way by convention, even though conception happens about two weeks later.
The conception method counts 266 days forward instead, which lands on the same estimated due date. Both are starting points. A first trimester ultrasound often gives a more precise date, and your provider will use that if it differs.
Understanding the trimesters
Pregnancy is usually split into three stages, each with its own rhythm:
- First trimester, weeks 1 to 13: early development and, for many, fatigue and nausea.
- Second trimester, weeks 14 to 27: often the most comfortable stretch, with first movements felt.
- Third trimester, weeks 28 to birth: rapid growth as the baby prepares to arrive.
The week count shown here is gestational age, measured from the last period, which is how clinics describe how far along you are.
How accurate is a due date?
A due date is an estimate, not an appointment. Only about 1 in 20 babies arrive on the exact date. Most are born in the two weeks before or after, and a full-term birth is anywhere from 37 to 42 weeks.
Cycle length, the timing of ovulation, and ultrasound measurements can all shift the estimate. Treat the date here as a friendly target and rely on your obstetrician or midwife for the figure that guides your care.
Frequently asked questions
- How is my due date calculated from my last period?
- The calculator adds 280 days, which is 40 weeks, to the first day of your last menstrual period. This is known as Naegele's rule and is the standard clinical starting point for dating a pregnancy.
- Why does pregnancy start counting from my last period?
- It is a practical convention. The date of conception is often unknown, but the start of the last period is usually easy to recall, so clinics measure gestational age from there. It places the count about two weeks ahead of conception.
- How accurate is the estimated due date?
- It is a good estimate but not exact. Only around 5 percent of babies are born on their due date, with most arriving within two weeks on either side. An early ultrasound can refine the date.
- Should I use the last-period or conception method?
- Use the last-period method if you know when your last period began, since it matches how clinics date pregnancy. Choose the conception method only if you are confident of that date, such as with a known ovulation or treatment date.
- Can this calculator replace my doctor?
- No. It offers a quick estimate for your own interest. Your obstetrician or midwife confirms dating with exams and ultrasound and is the right source for any decision about your pregnancy and care.