How to calculate your baby due date

If you’re pregnant, you probably want to find out when you’re going to give birth. Don’t wait any longer and calculate your baby due date!
You’ve just found out that you’re pregnant? Congratulations! You are probably itching to know when your baby will finally arrive. However, there’s no need to get annoyed at your calculator or give yourself a headache trying to multiply, substract and add a whole load of numbers just to find out when your delivery date will be. We’ve made things much easier for you by explaining how to calculate your pregnancy due date so that you can get ready for your baby’s arrival.

How is my baby due date calculated?

Unless you’ve undergone fertility treatment to become pregnant (such as in vitro fertilisation or intrauterine insemination) or you’ve been tracking your ovulation, chances are that you won’t know precisely the days you were ovulating and/or when you conceived. Luckily, to find out your baby due date, all you need to know is the day that your last period started.


Young pregnant woman holding a calendar for the birth of a child on a dark background


By the way, if you’re trying to conceive, it’s incredibly useful to keep track of your menstrual cycle. You can, for instance, take your temperature with a basal body thermometer or an ovulation predictor kit (also called OPKs or ovulations tests) to know when you’re ovulating. Not only will this allow you to know the time at which you are most fertile (your fertility window), it will also be a great help when it comes to calculating your due date later on.
To estimate your delivery date, you need to know the length of pregnancy. The average pregnancy lasts between 37 weeks (259 days) and 42 weeks (294 days) from the first day of your last period. However, as no one can know in advance how long the gestation period will be exactly, the estimation is based on a 40-weeks pregnancy.
The baby due date is also calculated according to the length of the cycle. Although most women have a cycle of 28 days, some have a shorter or longer cycle. The majority of women ovulate about two weeks after the first day of their period, when it is possible for conception to occur.

Calculate your due date now and find out when your baby will arrive!

To calculate your due date, you need to know two things: the date of the first day of your last period, as well as the approximate length of your cycle. If, however, you don’t know the length of your cycle, you can base your calculation on the average 28-day cycle.
Once you have this information (and if you have a 28-day cycle) add 280 days (which corresponds to forty weeks) to the first day of your last period. For example, if your last period started on the 9th of November 2017 and you add 280 days, your baby due date will be the 16th August 2018. This gives you plenty of time to prepare for your little one’s arrival!
If you want to estimate the date of delivery without having to search everywhere for your good old calculator, you can use the pregnancy calculator of Emma’s Diary. You just need to enter the date of the first day of your last period, adjust your cycle length if you know it (otherwise the estimate will be based on a 28-day cycle) and click on “calculate”. The results will appear instantly!

I have calculated my baby due date: what are the odds of my baby arriving on the exact same date?

Unless you’ve conceived via a fertility treatment such as IVF (where you can know the date of conception precisely), the calculated due date is rarely one hundred percent accurate – it’s just an estimate. You won’t know for sure what day your little one will arrive. Only around 4-5% of children are actually born on this predicted date, which corresponds to one in twenty women. Therefore, there is a chance that you will be going into labour one, two or even three weeks before or after the due date you’ve predicted.
Nevertheless, having an estimate of the due date serves to prepare you psychologically for your new life, labour and delivery, as well as for the arrival of your baby in a more practical sense (purchasing baby items, packing your hospital bag, etc.)

Will I receive a more accurate estimate?

You will probably have your first prenatal ultrasound scan during your first trimester, generally between the 10th-14th week of pregnancy. Your gynaecologist will make another calculation of your delivery date by looking at the development and size of the foetus. The predicted date is still an estimate, but more precise.
In case the two predicted due dates are different from each other by a week or more, the ultrasound scan is usually considered more accurate.

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