Are you talking humans here? Because 40 weeks is exactly the length of time it takes for a human baby to develop. However , our first child was 43 weeks in the making( and it took an XRAY for them to make sure she was still in there ) and the second pregnancy was 42 weeks - but that was 7 lbs + twins who were breach .
A normal human pregnancy lasts between 38 and 42 weeks. This is completely normal. It averages at 40 weeks but if you give birth at 42 it's still a normal delivery time. Most women say they are late (eg 2 weeks late with a 42 week pregnancy) but in fact they are right on time for their body.
babies turn up when they want to so don't worry. You can encourage the baby's arrival by a hot curry and a good rogering. (for her, not yourself :-) - worked for Mrs Vader!