Baby In Womb Royalty-Free Stock Image

It takes just 38 weeks for a fertilised egg to grow into a baby. However, we say that a human pregnancy is 40 weeks long. This is because doctors calculate the 'estimated date of delivery' of the baby from the beginning of the mother's last menstrual period - on average, two weeks before fertilisation. Only about a third of all fertilised eggs grow into a baby: most are lost before they are three weeks old.