After an effective sexual intercourse, hundreds of millions of spermatozoa that have made it to the uterus, rush through the fallopian tubes towards one single egg cell that has matured during the monthly cycle. Only one spermatozoid, the fastest and most resilient of the many millions of rivals, can fertilize the egg cell. The meeting process takes place in the ampulla of the uterine tube. The fertilized egg begins to divide rapidly. After 7–9 days of intensive cell division, the embryo already consists of several hundred of cells. In order for the embryo to become a fetus, it must reach the uterus and implant itself into the uterine wall. The placenta which is the mediator between the mother's body and the developing fetus, plays a huge role in the transformation of the embryo into the fetus. Four weeks later, an organism consisting of millions of cells arises from a single fertilized cell. During the second month of development, the embryo, despite its small size, already carries the features of a human being, since by the end of the eighth week the main organs and systems are outlined, and the embryo turns into a fetus. Fetal period lasts from the 9th week until the birth.