you add on the first two weeks as you weren't pregnant then but its to do with the total cycle length x
Why is there a difference between my official week of pregnancy and the length of time my baby has been developing?
You may notice that there is a difference of about 2 weeks between the length of time your baby has been developing and your 'official' week of pregnancy. We haven't gone crazy; there is a method to our madness!
We actually calculate weeks of pregnancy by counting from the first day of your last menstrual period, which we assume was exactly 40 weeks before your due date. As you don't ovulate (and therefore can't conceive) until approximately two weeks after the start of your menstrual cycle, the length of time that your baby has actually been developing is two weeks less than the number of weeks you have been officially pregnant. To put it in a more straight forward way - when you reach your 40th week of pregnancy your baby will only have been developing for 38 weeks.
This is a method commonly used to calculate EDDs by those in the medical profession, as for most it is usually much easier to recall the first day of your last menstrual period than to know exactly when you ovulated. So, its much simpler for everyone to count forward from their last period, although this does mean that technically for the first two official weeks of your pregnancy you weren't actually pregnant - bizarre, but that's why there is a difference.
