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Your period

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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.

Why it can be wrong

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.