In the popular mind, pregnancy lasts nine months. In medical terms, it lasts 40 weeks.
However, just to make things more confusing, you count the months from a different date than you calculate the weeks. And neither of these calculations takes account of the time the baby has actually existed - that is yet a third way of reckoning.
As an example, let's consider a woman whose last menstrual cycle started on the first of May last year. That is to say, May 1, 2000 * was the first day of her last period before she got pregnant.
Calculation by Months
The calculation by months starts from one week after the beginning of the last cycle. In this example, we count from the 8th of May. By that calculation, the baby was due on the 8th of February 2001.
Calculation by Weeks
Calculations of the weeks of pregnancy, as used by the medical profession, start from the first day of the last period. For our example, calculation by weeks starts on the first of May 2000. Forty weeks from then, or 280 days, leads to a due date of 5th of February 2001 - slightly different than the monthly calculation.
Existence of the Baby
Neither calculation actually takes account of the time egg meets sperm. An average woman has a cycle of 28 days, and ovulates halfway through that time. Since the egg only lasts about 24 hours after ovulation, that means that the woman in our example ovulated on May 14, 2000 and conceived no later than May 15, 2000, when she was already one week (counting by months) or two weeks (counting by weeks) pregnant.
I suppose it gives parents to be something to think about. Keeps them from worrying about what the baby will be like.
* I confess - I chose May for these calculations because it gives the widest divergence between the two due dates. For 7 months of the year (January, February, June, August, September, October and November), the calculations come out the same.