The phases of Development of the Fetus , MD, Saint Louis University Class of Medicine Once per month, an egg is released from an ovary into a tube that is fallopian. After sexual activity, sperm move through the vagina through the cervix and womb to your fallopian tubes, where one sperm fertilizes the egg. The fertilized egg (zygote) divides over and over over over and over repeatedly because it moves along the tube that is fallopian the womb. First, the zygote turns into a ball that is solid of. Then it turns into a ball that is hollow of called a blastocyst. In the uterus, the blastocyst implants within the wall surface of this womb, where it develops into an embryo attached with a placenta and surrounded by fluid-filled membranes. The placenta and fetus have been developing for 6 weeks at 8 weeks of pregnancy. The placenta kinds tiny hairlike projections (villi) that stretch to the wall surface for the womb. Arteries through the embryo, which move across the cord that is umbilical the placenta, develop when you look at the villi.