When there's between 16 and 32 cells, the fertilized egg is now referred to as a morula . At this stage, it's referred to as a blastocyst . Trophoblast cells develop into the placenta. The innermost cells of the blastocyst develop into the embryo. About 7 days after fertilization, the blastocyst travels down fallopian tubes and enters the uterus.
Trophoblast cells invade the walls of the uterus and begin to proliterate.
The uterus becomes thicker and larger and contributes cells to the developing placenta. The developing fetus is in a fluid-filled sac known as the amniotic sac. At this point, hormone contributions are necessary for survival of the fetus. The most important hormones are estrogens and progestins. These levels increase during pregnancy until right before delivery of the baby.