How does the due date calculator work?

There are very few things in life that can compare to the rush of excitement when the pregnancy test goes positive. A million thoughts rush through your head in one minute. And when the heart flutters calm down and the brain kicks in one thought surfaces to the top and that is – planning. One of your first questions will be when is my baby due?

Our three-step pregnancy calculator in the header image will help you find your pregnancy due date:

  1. Click select date and enter the first day of your last period

  2. Check the date you have selected and click calculate

  3. The calculator will show your due date. Click take me there to find out everything about your week of pregnancy

Sadly, babies don’t work like clockwork, so the due date is an estimate rather than a fixed date.

woman smiling at positive pregnancy test

How to calculate your due date?

Pregnancy is not calculated from the first day of your missed period, as many might think. In fact, on the first day of your missed period you are already four weeks pregnant. Unless you have been tracking your ovulation, it is very difficult to know for sure exactly when you ovulated and conceived. That’s why your due date is calculated based on the first day of your last period.

Most women ovulate about two weeks after the first day of their period and conceive shortly afterwards. Most pregnancies last 40 weeks, our due date calculator uses this information to work out your due date.

It is important to note that it is an estimate. Unfortunately, babies work on their own timescale so use the date as an indicator rather than a fixed date.

Will my baby be born on my due date?

Unless you have been having fertility treatment it is very difficult to know the exact date of conception - and even if you did know, pregnancies can last anywhere between 37-42 weeks. In fact, only 4% of babies are born on their due date. It is likely though that your baby will be born in the few weeks either side of your due date. Knowing this can help you to get planning - from preparing for labour and birth plan to baby showers and hospital bags.

Will the due date change when I have a scan?

Your due date might change when you have your first pregnancy scan. You usually have your first scan at 12 weeks, however it can happen anytime between week 8 and 14. It is known as the dating scan and it will give you a more accurate idea of when the baby is due, usually it’s not as far advanced as the date of your last period suggests. The due date is calculated based on the size and development of the baby.

father kissing pregnant mother’s baby bump

Time to start pregnancy planning

You’re about to have many amazing milestone moments, from babies first selfie on the antenatal scan, to preparing your birth plan and maybe some weird and wonderful pregnancy cravings.

First things first though, if you haven’t already done so you should book an appointment with your doctor. They will confirm your pregnancy with a blood test and start your antenatal care.

For everything else try the Mother of all Checklists web app. This is the ultimate pregnancy app for planning, with checklists on everything from what to ask at the first antenatal appointment, to what to pack in your hospital bag and everything in between. It really is the best planning tool for pregnancy, and we are rather proud of it!

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