There's no pregnancy week by week rule book, but typically you officially find out the date your baby is due between eight and 14 weeks, when you go for your ultrasound scan. Your doctor will then be able to tell you when you can expect your new bundle of joy - but remember, it's not a perfect science and babies have a habit of making their mums wait for their grand debut!
If you've yet to see your doctor and are keen to know roughly when your little one will arrive then you could count backwards and try to work it out manually, but there's not need when you've got us to do the work for you! Our handy due date calculator tool will help you work out exactly when your baby's due. Just tell us the date of your last period and we'll tell you when you can expect your new addition to the family - simple!
Try our due date calculator now by entering your details below!
Your due date is important as your doctors and midwives need to have an idea of when your baby will arrive so they can tell if they are early and are likely to need special care, or if you're so 'overdue' the baby could be at risk. But because the methods used to calculate EDDs can be inaccurate, lots of mums-to-be have Caesareans or are induced unnecessarily.
Even more face the frustration of sitting at home waiting for a baby who has no intention of putting in an appearance for days to come. So how does the medical profession come up with EDDs, and why are they so rarely on the mark?
Getting your due date from your doctor
When you see a doctor or midwife to confirm your pregnancy, you'll be asked the date of the first day of your last menstrual period (LMP).
Your due date is nine months and seven days from this day. This is the most common method of working out an EDD.
The problem with this system is that it's based on the assumption that your menstrual cycle is 28 days long, and that you ovulate on the 14th day of your cycle. But if there's one thing that can't be ruled by averages, it's the female reproductive system.
How your last period affects your due date
Every woman has a different cycle and will ovulate at a different stage. If your cycle is shorter than 28 days, your due date should be slightly earlier; if it's longer, it will be slightly later.
If you're absolutely sure of the precise date you conceived, tell your doctor, as he may calculate your due date from this instead.
But this method obviously doesn't work for women who don't know the date of their last period or for those who've conceived through IVF or other kinds of fertility treatment, as they don't depend on your natural cycle.
Having a 'dating scan'
In some areas of the country, you will have a 'dating scan' at around 12 weeks. Not only will this confirm your pregnancy, but it will also give the sonographer a chance to check the size of your baby and see if it corresponds with the due date you have been given, based on your late menstrual period (LMP).
There are charts that show the average size of foetuses at each stage of gestation, and if the size of your baby is bigger or smaller than normal for the stage you think you're at, your due date may be adjusted.
This doesn't mean your baby is abnormal or isn't growing properly, just that your original due date may have been a bit early or late. This could be because your menstrual cycle is longer or shorter than average,or that you ovulated a little earlier or later in your cycle.
If you've had IVF or other fertility treatment, you'll be given an early scan to confirm the pregnancy and predict a due date.
This method is more accurate than the one worked out according to your LMP but, again, it's based on averages, in this case the average size of unborn babies at each stage. And just as some little ones are bigger than others at birth, some are naturally bigger or smaller than others at 12 weeks' gestation.
The other problem is that not all women are offered dating scans, many don't have a scan until close to 18 weeks. Scans up to 12 weeks' gestation accurately predict an EDD, but after 13 weeks they're only accurate to within about two weeks.
What to do once you've got your due date
Once you've worked out your approximate due date with our calculator or your doctor you can get down to the exciting things, like planning how their nursery will look and picking our some of your favourite baby names. You could consider anything from Disney names to the most popular choices, depending on what you fancy.
And then of course, there's also the fun job of your pregnancy announcement. Once you've settled into your pregnancy there are so many exciting ways to let friends and family know your happy news.
Did your baby arrive exactly on your due date? Tell us your experiences of being early, late or exactly on time with the birth of your baby in the comments section below or on our Facebook page.