Wait — what happened to weeks one, two, and three of the pregnancy? Trick question! You're not actually pregnant during weeks one and two (those are freebies, since we start counting on the first day of your last period). At week three, conception probably just occurred and the fertilized egg is implanting
By week four, your baby, or more accurately your little collection of cells, is 0.07 inches, or about the size of a grain of sand. So cute!