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Due dates

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So few women actually give birth on their EDD (estimated due date) that it can make you wonder why doctors bother working it out. A third of babies are born seven days eitherside of their due date, but around 40 per cent arrive more than a week after.

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 are 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?

Use our ovulation calculator to work out your approximate duet date and then read on to find out more about why your period affects the result...

By Siobhan O'Reilly