How your due date is calculated
This pregnancy calculator uses Naegele's rule, the same method your midwife uses at your booking appointment: it adds 280 days, or 40 weeks, to the first day of your last menstrual period (LMP). That is the same as adding 7 days, subtracting 3 months and adding a year. For example, if your last period started on 10 January, your estimated due date is around 17 October.
The rule assumes a 28-day cycle, with ovulation around day 14. If your cycles are longer or shorter, the calculator moves the date by the difference: with 32-day cycles your due date is 4 days later; with 25-day cycles it is 3 days earlier.
Weeks pregnant: why they are counted from your last period
Pregnancy weeks (gestational age) are counted from the first day of your last period, not from conception. So when a test is positive on the first day of a missed period, you are already about 4 weeks pregnant, even though the embryo is only about two weeks old. Scan reports use the same system, written as weeks + days (for example 12+3).
To see what happens each week, have a look at our pregnancy calendar week by week.
The three trimesters
- First trimester: up to week 13. Nausea, tiredness and your first scan.
- Second trimester: weeks 14 to 27. Often the easiest stretch, and you start to feel your baby move.
- Third trimester: from week 28 until birth. Your baby gains weight and it is time to get ready.
Our guide to pregnancy symptoms by trimester explains what is normal in each one and when to get checked.
Why your due date is only an estimate
A full-term pregnancy lasts between 37 and 42 weeks, and few babies arrive on their due date. The date of your last period is not always exact either: irregular cycles, not remembering the day, or getting pregnant straight after stopping the pill or while breastfeeding all throw the calculation off.
That is why the dating scan in the first trimester measures the embryo and, if the difference is significant, your due date is adjusted. From then on, the scan date is the one that counts. This calculator does not replace care from your midwife or doctor.
Frequently asked questions
How long is a pregnancy?
About 40 weeks from your last period, a little over 9 months. Your baby is considered full term from 37 weeks.
Can I work out my due date from conception?
Yes: add 266 days (38 weeks) to the date of conception. You get the same date as from your period, because conception usually happens about two weeks after it starts.
What if I had IVF?
With IVF the transfer date and the age of the embryo are known, and your clinic will calculate your due date from them.
Not sure yet if you are pregnant?
Read when to take a pregnancy test to get a reliable result. If you are trying to conceive, our ovulation calculator shows your fertile days.
Getting ready for your baby
With a date in mind, it is a good moment to start on your baby list and pack your hospital bag, ideally by week 36. And even if you plan to breastfeed, many mums-to-be choose a bottle before the birth: the Happymami bottle is designed from the shape of your breast, so your baby accepts it more easily and breastfeeding is not disrupted.
