I am currently reading The Official Lamaza Guide: Giving Birth with Confidence by Judith Lothain and Charlotte DeVries. Here is a quote that I enjoyed.
"Though you and your care giver might calculate a due date forty weeks from your last period, bear in mind that only about 5 percent of babie arrive on ther due dates. Many facotrs make due dates mard to pin down...And some babies just take more or less than forty weks to be ready for the outside world.
Imagine making a batch of popcorn. Most of the kernels pop during a few noisy moments But there are lots of early and late poppers too. All the popped kernels are perfect- but each need a slightly different amount of time to be fully cooked. The same is true for babies."