The menstrual cyle is the shedding of the uterine lining that would otherwise have housed a growing embryo in pregnancy. When a woman becomes pregnant, that uterine lining is necessary to house the new embryo and is retained inside the womb. Since the lining doesn't shed, the woman doesn't menstruate during her pregnancy.
The Menstrual Period
A woman's normal monthly cycle begins when an ovum starts to mature in one of her two ovaries, and the blood-rich lining of the uterus thickens due to the hormone progesterone produced by the ovaries. After about two weeks, ovulation occurs, which means the ovaries release an egg into the fallopian tubes. Pregnancy occurs when the egg is fertilized by sperm, becomes an embryo and attaches to the uterine wall. If the woman becomes pregnant, the embryo remains in the uterus and starts to grow. If she does not, her progesterone levels drop, and the thickened lining of the uterus (and the egg) is shed from the uterine wall in the menstrual period.
When a woman is pregnant, her hormone levels rise, maintaining the thickened lining inside the uterus and preventing it from being shed. This is important because the developing embryo inside her is embedded in the uterine lining that would otherwise have become the menstrual blood. If the woman were to have a period, the embryo would be washed out of the uterus along with the uterine lining and would never be able to grow. Instead, when the egg is fertilized, progesterone levels remain high, keeping the uterine lining in place. This is why amenorrhea (lack of a period) is one of the telltale signs of pregnancy, although other conditions can also cause amenorrhea.
Bleeding During Pregnancy
Bleeding can occur during pregnancy that may be confused with a menstrual period. Implantation bleeding is the most common type of bleeding. When the fertilized egg, now an embryo, implants into the uterine lining, it sometimes causes a little blood to shed. This can be mistaken for a woman's normal period, especially since implantation often occurs at about the same time she would expect her next period. Also, the first symptoms of pregnancy are often the same as symptoms of an approaching period. Other bleeding during pregnancy, such as early miscarriage, could conceivably be mistaken for a period, but this typically occurs only when the woman does not know she is pregnant.