Due date calculator
Enter the first day of your last period to see your estimated due date, how many weeks pregnant you are today, and the range in which most babies are born.
How it works
The Institut national de santé publique du Québec says to estimate a due date by counting 280 days, or 40 complete weeks, from the first day of your last period. That is the method here.
It assumes a cycle of 28 days. If yours is longer or shorter, the calculator moves the date by the difference. The NHS says to adjust for the length of your cycle and does not say how, so this is the simple way to do it, and your midwife’s date comes first.
The range
The NHS says a pregnancy normally lasts from 37 to 42 weeks from the first day of your last period. The Quebec institute says most babies are born between 39 and 41 weeks. The calculator shows both ranges as dates, next to your estimated due date and the weeks you are today.
A worked example
Say your last period began on January 1, 2026, and your cycle is 28 days. Counting 280 days gives Thursday, October 8, 2026. Most babies are born between October 1 and October 15. The normal range runs from September 17 to October 22.
With a 35 day cycle, the same last period gives October 15.
Why your doctor’s date can differ
The Quebec institute calls the expected due date only an approximate date. An ultrasound before 20 weeks gives a good idea of the date, plus or minus 7 to 10 days. The NHS says the scan that a midwife offers at about 12 weeks can estimate more accurately how many weeks pregnant you are. When your midwife or doctor gives you a date after a scan, use that one.
This calculator is not medical advice.
Sources
- Institut national de santé publique du Québec: length of pregnancy and due date, read on 2026-10-31.
- NHS due date calculator, read on 2026-10-31.
Questions
How is a due date worked out?
The Quebec public health institute says to count 280 days, which is 40 complete weeks, from the first day of your last period. This calculator does exactly that.
Why is the date different after my scan?
The expected date is only approximate. The Quebec institute says an ultrasound before 20 weeks gives a good idea of the date, plus or minus 7 to 10 days. Use the date your midwife or doctor gives you.
What if my cycle is not 28 days?
The method assumes 28 days. The calculator moves the date by the days your cycle differs from 28, so a 35 day cycle gives a date a week later. The NHS says to adjust and does not say how.
How many babies are born on the due date?
The Quebec institute gives no share and says the date is only approximate. It says most babies are born between 39 and 41 weeks, and that a baby could arrive anytime from 37 to 42 weeks.
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