A full-term baby averages a little over 19 inches in length. This remarkable growth occurs over the course of nine months. During the last four weeks of pregnancy, body growth slows as the baby puts on weight â mostly in the form of body fat â in preparation for birth. Time Frame. The last four weeks of pregnancy are weeks 36 through 40. During this time, the baby gets ready to make her entrance into the world. Space is tight, so she doesnât move around as often. Her lungs finally mature, and in most cases, she moves into a head-down position to make the birth process easier.