Actually, biology doesn't say that united a sperm and an egg gives you a human being. Uniting a sperm and an egg gives you a fertilized egg which initially has two seperate nuclei each with only half the chromosomes needed to produce a human offspring. There are a multitude of events that must occur before a human being is produced. And the fetus is most definately part of the mother. It is her blood and her body which allow the fetus to develop. In fact, if the mother's body does not recognize the fetus as part of itself, it will reject the fetus in a spontaneous abortion.