Calculating your baby’s due date
Finding out your baby’s due date is an exciting moment and helps you work out how far into your pregnancy you are.
Your 12-week scan, also known as a dating scan, will give you the most accurate estimated due date as the sonographer will carefully measure your baby to see how developed they are. But there are ways to estimate your baby’s due date before you go for your first scan.
Your midwife or GP will estimate your due date by asking you when the first day of your last period was. If you know the answer to this question, they will then count 40 weeks, or 280 days, from that date to give you your due date.
Don’t worry, they won’t be trying to work this out in their heads; most midwives have an obstetric or pregnancy wheel to help them calculate an accurate due date. You can also work it out yourself by counting ahead 40 weeks on a calendar or using an online pregnancy due date calculator.
Another way of calculating your baby’s due date is to subtract three months from the date of your last period and then add one year and seven days.
These calculations are based on an average menstrual cycle length of 28 days, but many of the online due date calculators allow you to adjust your cycle length. If your periods are irregular, just choose your average cycle length.
If you became pregnant using IVF, your doctor will calculate your estimated due date based on the date your embryo was transferred into your womb.
How do I work out my due date if I don’t know when my last period was?
If you’re not sure when your last period was, it is more difficult to work out your baby’s due date without an ultrasound scan. Speak to your midwife for advice, and she may recommend you go for a scan to get an accurate idea of how many weeks along your pregnancy is.
You may also be able to work out your estimated due date if you know when the likely date of conception was. If you know a specific date your baby would have been conceived for you to have become pregnant, you can add 38 weeks to calculate your estimated due date.
How accurate are estimated due dates?
Your due date will give you an exciting entry for your calendar, but it probably won’t be the day your baby is born. Less than 5% of babies actually arrive on their due date; most are born at some point between 39 and 41 weeks of pregnancy.
The estimated due date you or your midwife work out at the start of your pregnancy may well be adjusted when you go for your dating scan at around 12 weeks.
Why is the due date worked out from the date of your last period?
As your due date is estimated from the date of your last period, you are considered to be four weeks pregnant, just two weeks after your baby was conceived. This can be confusing, but it is easier for most people to remember when their last period started than to work out when their baby might have been conceived.