Now firmly ensconced in your uterus, the blastocyst quickly sets about becoming an embryo. For the next several days, it will double in size every day. Its cells differentiate into two distinct types: those that will form the embryo, and those that will develop into the yolk sac and, eventually, the placenta.

At about day 10 after fertilization, three layers become apparent in the embryo cells. The endoderm, or inner layer, develops into your baby’s bladder, lungs, digestive system, and other organs. The mesoderm, or middle layer, becomes the heart, reproductive organs, bones, and muscles, and other tissues. The ectoderm, or outer layer, forms many organs and tissues including the hair, skin, nails, and nervous system. The embryo becomes pear shaped, broader at the end that will become the head. A longitudinal line called the “primitive streak” appears, the central axis around which tissues, organs, and eventually systems will form. The line that runs through the broad end of the embryo indicates where the brain will divide into two hemispheres.