American Medical Association Family Medical Guide - American Medical Association [564]
Progesterone
After ovulation, the ovaries produce progesterone, which stimulates the lining of the uterus to thicken with blood in preparation for the implantation of a fertilized egg. During pregnancy, the placenta (the organ that links the woman’s blood supply to that of the fetus) produces progesterone to maintain a healthy pregnancy.
Testosterone
Testosterone is the primary male sex hormone, responsible for the development of the secondary sex characteristics in boys at puberty. The hormone also has a role in maintaining sex drive in both men and women. Testosterone is produced in a man’s testicles and, in small amounts, in a woman’s ovaries.
Menstruation
Your body gets ready for the possibility of pregnancy each month during the years between the time you enter puberty and the time you reach menopause. Every 28 days or so, one of your two ovaries releases an egg, a tiny single cell that is invisible to the naked eye but that contains half of your genetic material. This process is called ovulation. The egg travels down a fallopian tube to the uterus, taking about 5 days to complete its journey. If you have intercourse shortly before or after the egg is released, your egg may be fertilized by your partner’s sperm inside the fallopian tube. Sperm can live for as long as 6 days, so if you have intercourse 5 days before the release of your egg or 24 hours after, a sperm will still be able to fertilize the egg.
A few days before ovulation, the lining (endometrium) of your uterus gradually thickens. This process prepares your body for the possibility of fertilization and pregnancy. The thickened lining will become the first food supply for your fetus. By the time a fertilized egg reaches the uterus, the lining is ready for the egg. Once the egg attaches itself to the lining, it starts developing into an embryo, and you are officially pregnant.
If the egg is not fertilized, the thickened, blood-filled lining of the uterus will begin to shed, along with the unfertilized egg, about 14 days after ovulation. The bloody fluid passes out of the cervix (the lower opening of the uterus) into the vagina and out of the body. This process, called menstruation, lasts an average of 5 days. During the next 9 days, another egg matures and grows in an ovary, starting the process of ovulation again.
The Menstrual Cycle
Day 1: The lining of the uterus (endometrium) begins to shed (menstruation).
Day 9: An egg ripens and the endometrium thickens.
About day 14: The ovary releases an egg into the fallopian tube.
Days 14-19: The egg travels down the fallopian tube.
Days 20-27: An unfertilized egg passes out of the uterus; the endometrium continues to thicken.
The entire menstrual cycle lasts an average of 28 days, although in most women it fluctuates by 1 or 2 days and sometimes more. Each stage of the menstrual cycle is controlled by a cascade of interrelated hormones produced by the hypothalamus and pituitary gland in the brain and by the ovaries.
Periods usually start between ages 11 and 14. The first period is called menarche. Periods may be irregular for the first couple of years because ovulation often does not occur regularly at first. Periods can also become irregular after about age 45 because of fluctuating hormone levels and variable ovulation. Periods finally stop permanently at menopause (see page 851), at the average age of 51.
Absence of Periods
The temporary or permanent absence of periods is known medically as amenorrhea. In some girls, periods fail to start at the normal age, usually between ages 11 and 14. This is called primary amenorrhea, which usually results from a normal but late onset of puberty but can also be caused by a hormonal imbalance or an abnormality in the reproductive system. Usually doctors do not look for abnormalities until a girl reaches age 16 without having a period, or 14 if she has not had any signs of sexual development, such as breast enlargement or pubic hair.
It is common to miss a period