Pregnancy due dates are not accurate. They are determined based upon Naegele’s rule, a system from the 1800′s created without any scientific research.
Multiple factors determine the gestational period of humans, and they vary based upon a woman’s ethnicity, height, weight, age, length of menstrual cycle, day of ovulation, and so forth. The current thinking is that it is time to move away from using Naegele’s rule and stop giving women a precise date.
It would be incredibly more useful to give women a month to focus upon, so that they are not needlessly worrying about missing what amounts to an arbitrary day, since only 3-4% of babies are born on the date given, and the same 3-4% chance exists for a baby to be born on any of the other days up to a week either before or after the due date given. Read Wikipedia for more details.
What is important for expecting parents to understand is how a women’s body functions, and know that it is not possible for a baby to miss it’s Birthday.
When a baby has completed development in the uterus, it will begin to secrete a hormone that interacts with the mother and begins the birth process. A number of other changes will begin to occur as part of the birth process. The mother’s cervix will soften, or become ripe. If you touch the tip of your nose you notice it is firm, that is an unripe cervix. But, it you touch the middle of your lower lip, you feel it is soft, that is a ripe cervix.
Similar to telling if a piece of fruit is ripe or ready to pick or eat. Most people do not wish to eat an unripe piece of fruit. It’s too hard. And an excellent farmer would never pick the crop before its time.
And there are many other ways to tell. Such as the baby descending, and so forth.
The body gives proper signals each day: when to eat, to drink, to sleep, to use the bathroom, if one is ill, etc. Most people are aware of these signals and act without fail. The body does not usually fail to make sure we get to the bathroom in time, nor do we miss many meals.
How can it be that our bodies signals do not work properly to let us know when our babies should be born?
The trust we have in our body to signal us to sleep, is the same trust we must have that our body will signal us when our baby is fully developed and ready to greet us.
Links about determining Pregnancy Due Dates: