The beginning of the male ejaculations and the female menstrual cycle are the signs of a person reaching sexual maturity. This means that the person is ready to have children. 

However, giving birth to a child would be too difficult a task for a pubescent girl. In addition, pubescent boys and girls are not mentally ready to take care of a child. 

Having children also requires that both the mother and the father are ready and willing to raise the child into maturity. 

When making love, a man and a woman are having sexual intercourse. In sexual intercourse, a man places his erect penis into the woman's vagina. When the man ejaculates inside the woman's vagina, millions of sperm cells are released. These sperm cells begin travelling towards the womb and the Fallopian tube. 

If the Fallopian tube contains a detached, mature egg cell, it can be fertilized by a sperm cell. During fertilization, the sperm cell penetrates the membrane of the egg cell and the two cells are fused together. 

The fertilized egg cell, which now contains genes from both the father and the mother, can now begin developing into a child.

The stages of fertilization:
1. An egg cell develops in the woman's ovaries. The mature egg cell is detached. 2. The vagina receives male sperm cells during sexual intercourse. 
3.–4. The sperm cells swim through the womb and reach the fallopian tube. 5. The fastest sperm cell fertilizes the egg cell.