When will my baby arrive?

Advice on the accuracy of your estimated due date and how close to this you can expect your baby to arrive.

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One of the first things everyone wants to know when they find out they're pregnant is when their baby will be arriving and with good reason, adjusting to the idea of becoming a parent (again or for the first time!), planning things like family visits and maternity leave and working out when you should have the nursery ready and your bag packed are all so much easier if you have a date to work around. However, while your healthcare provider is likely to give you a due date you shouldn't rely on this too much. In fact, what your doctor will work out is known as an 'estimated due date' (EDD) which may give you some idea of its accuracy!

How is my due date worked out?

Your doctor is likely to work out your EDD based on a rule that was developed by a Dr Nagele in the mid 19th century (yes that long ago!). He calculated that the average gestational period of a pregnancy is 266 days (which equates to 38 weeks) from conception, or 280 days (40 weeks) from the first day of your last menstrual period (LMP) and so formed a rule which calculates your baby's expected arrival based on this. You can do this calculation yourself by subtracting 3 months from, and then adding 7 days to, your LMP. For example, if the first day of your last menstrual period was 12th June, to work out your EDD you would have to take away 3 months (which takes you to 12th March) and then add on 7 days. This gives you an estimated due date of 19th March the following year.

How reliable is this?

While this will give you a good idea of the time around which to expect your baby's entrance into the world its not a definitive date as, because of several assumptions made, there is a large margin of error so I wouldn't go putting any money on it yet.

Firstly, the Nagele rule works on the assumption of a 28 day menstrual cycle with ovulation on day 14; if your typical cycle is, on average, shorter or longer than this, or you do not ovulate exactly 14 days from the start of your cycle (few women actually fit this calculated norm) then the EDD will be less precise and is likely to be out. Additionally, the rule also assumes a 40 week gestational period, although you are not technically pregnant for the first two weeks as ovulation and therefore conception has not yet occurred. Again this gestational period is an average based on Nagele's calculations back in the 1850's and is liable to vary. Also, because of enhanced prenatal care and furthered medical knowledge there is a developing trend towards women carrying their baby's for longer, which again may mean you overshoot the mark a little.

For women who are unsure of the date of their LMP or who suffer from irregular cycles and so are unable to use the Nagele rule, doctors are able to use ultrasound to work out an EDD. Most women will undergo examinations and ultrasounds throughout their pregnancy too and these can be more accurate in predicting length of gestation as they take into account factors such as uterus size and foetal development.

What factors influence the due date?

Although there is surprisingly little research into this area, Robert Mittendord of the University of Chicago Medical Centre identified 16 factors thought to play a causal role in determining whether a baby is born on its predicted due date. For instance, he found that Caucasian women tend to have significantly longer gestation periods than their African or Asian counterparts, with an extra 8 days of pregnancy being quite regular. Additionally, women expecting their first child tend to have significantly longer pregnancies than those expecting their second or subsequent children. Other factors such as age and prenatal diet also seem to play a role.

Is there any way I can know for sure when my baby will arrive?

Unfortunately, at the moment although we can make educated guesses as to when babies are likely to arrive, we have little way of knowing exactly when to expect them. In fact, research shows that only 5% of babies are actually born on their predicted due date, 70% tend to arrive after this whereas 30% come into the world 'early'. As a little reassurance though around 80% of healthy babies are born within the two weeks before, or the two weeks after their EDD, so mentally preparing yourself for your baby's arrival sometime between the 38th and 42nd weeks of your pregnancy is a good idea. In fact some doctors have even recommend estimating a month of arrival rather than a specific day.

It can be hard not to get too attached to the due date specified by your doctor or midwife as it is only an estimate and they themselves may alter it over the course of your pregnancy as they monitor your baby's development. Some women do find it quite stressful if their due date comes and goes without a peep from baby and this isn't usually helped by the onslaught of 'when's you're baby due?' questions that will no doubt follow you throughout your pregnancy. However, by preparing yourself well in advance (so if baby does decide to make an early show you have everything sorted), keeping busy, spending time looking after your body and enjoying some 'me' time before your baby's arrival you'll be able to take it all in your stride even if your EDD (and your baby's sense of time keeping) is a little off!

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