Pregnancy occurs when a male and female have sexual intercourse and the male ejaculates semen from his penis into the females vagina. Semen is the white, sticky fluid that contains sperm, which can fertilize a female's egg.
Next, the sperm travels through the female's vagina, passes through her cervix and uterus and then into her fallopian tubes where an egg may sit after being released from the ovaries.
If the female has recently ovulated, then sperm fertilizes the egg and causes a pregnancy. The fertilized egg travels down the fallopian tube and plants itself in the uterus. Once this has happened, pregnancy has taken root.
If the egg and sperm fail to unite, then the egg dies. Without a pregnancy in place, the lining of the uterus, which is composed of blood cells, sheds. This process of shedding is called menstruation.