After implantation, the blastocyst begins to turn into an embryo, which is a more advanced stage of development. The process of becoming an embryo is complete when the amniotic sac and the placenta form. The fetus develops inside the amniotic sac, floating in amniotic fluid. The placenta is attached to the wall of the uterus. It delivers oxygen and nutrients from the mother's bloodstream to the fetus, during the pregnancy.