Due Date Calculator
Calculate your due date based on the first day of your last menstrual period, and see which pregnancy week and trimester you're in right now.
How to use the due date calculator
- Select the date of the first day of your last menstrual period.
- Adjust the cycle length if it differs from 28 days (the default).
- The calculator shows your due date, which pregnancy week you're in today, and which trimester.
How the due date is calculated
The calculator uses Naegele's rule, the most common method for estimating a due date, adjusted for cycle length:
Pregnancy is counted from the first day of your last menstrual period, even though conception itself typically happens around two weeks later. A full-term pregnancy lasts on average 40 weeks from this date. The first trimester runs through week 13, the second trimester through week 27, and the third trimester runs until birth.
Frequently asked questions
Why is pregnancy counted from the last menstrual period instead of conception?
The exact time of conception is rarely known with certainty, while the date of the last menstrual period is easier to pin down. That's why healthcare providers use this date as the standard starting point.
What if my cycle isn't 28 days?
The calculator adjusts the due date based on the cycle length you enter. A longer cycle gives a later due date, a shorter cycle gives an earlier due date.
Is an ultrasound more accurate than this calculation?
Yes. An ultrasound measurement early in pregnancy is considered more precise than calculating from the last menstrual period alone, and is therefore often used to adjust the due date by your midwife or doctor.