The fertilized egg - zygote first divides into two cells. Then each of them divided again and again. This eggs becomes after some process. This embryo travels down the oviduct to uterus. Then it sinks to uterus. This process is called Implantation.

Implantation is the process in which the fertilized egg or embryo attaches itself to the walls of the uterus.

After 8 weeks, when all organs are formed, the embryo is called a Fetus. One of the first organs to form is the heart. The embryo becomes enclosed in fluid filled sac called amnion sac or water sac. It protects it from damage and prevents unequal pressures acting on it. The fluid that is inside the amnion sac is called amniotic fluid.

The oxygen and food that is needed to keep the embryo alive are obtained from the mother's blood through a structure called placenta. This placenta is attached to the lining of the uterus.

The structure that connects placenta to the embryo is called umbilical cord.

Blood entering the placenta from the mother doesn't mix with the embryo’s blood.