4 Weeks Pregnant

Some women swear they feel the fertilized egg hunker into the lining of the uterus and get cozy. (The official term: implantation. The feeling: A twinge similar to ovulating). The rest of us won't have any notion we're pregnant for at least another week. While we're oblivious, Baby's getting busy. During week 4 the blastocyst burrows into the womb and separates into cells that will become the placenta and cells that will become the baby. The Great Divide isn't all that happens this week. The layers of the embryo itself actually start to differentiate into specialized parts. The outer layer will become baby's hair, skin, eyes, and nervous system; the middle layer will soon be the heart, reproductive organs, bones, muscles, and kidneys; and the inner layer the baby's liver, lungs, and digestive system.