Typical pregnancy is 40 weeks, right? Everyone agree? Let's do the math. 40*7/30 = 9.33 months. But you're only pregnant for 9 months, right? Where'd that other .33 months come from? Oh yeah, it's from the time before implantation that is still calculated into the length of the pregnancy for purposes of determining gestational age.
No one is going to argue that you're not literally pregnant two weeks before you get pregnant. But for determining the length of gestation, the beginning of your current menstrual cycle is how they figure out how far along you are.
Here's an excerpt I took right out of my book, Mayo Clinic Guide To A Healthy Pregnancy
