A mother's reproductive system provides the underlying support for her baby's growth, so you'll need a basic knowledge of this system to fully understand what's happening as your child develops.

Fertilization
Pregnancy begins when the male sperm fertilizes the female egg. This fertilized egg eventually develops into a baby. The egg becomes embedded in and grows within the hollow, muscular organ known as the uterus, or womb. During pregnancy, the uterus grows dramatically, attaining a weight about 20 times the original weight and the capacity for supporting a full-term infant.

Fetus and placenta
The developing baby is known as a fetus. The lifeline between you and your unborn baby is the placenta. The placenta is essentially a blood-filled organ bounded on one side by you and on the other by the fetus. The placenta allows oxygen and nutrients to pass from you to the fetus and passes waste products from the fetus into your body for disposal.

Umbilical cord
The placenta is connected to the fetus by the umbilical cord, which is made up of 3 blood vessels. Two of the vessels carry blood from the fetus to the placenta for cleansing; the third carries oxygen and nutrients to the fetus.

Amniotic fluid
The other vital part of your developing baby's life support system is the amniotic fluid. This fluid supports the fetus while allowing free movement, helps maintain a constant temperature, cushions and protects the fetus, and receives materials excreted in fetal urine.

Be prepared for the many changes that will occur in your body during pregnancy:

Breasts
When you are pregnant, your breasts will get bigger. You may need a bra 2 sizes larger than usual. Your breasts will feel firm and be more tender, and you may experience an occasional tingling sensation or shooting pain. You may find that the veins in your breasts are closer to the surface, more numerous, and larger than usual. The nipple and the areola—the area around the nipple—may change, too. The small glands around the nipple may become raised and lumpy and change color.

You may notice that your nipples are leaking or that if you put any pressure on your breasts a fluid appears. This fluid is just a type of milk called colostrum.

Uterus
During pregnancy, your uterus will grow, becoming capable of holding 1,000 times more than when you are not pregnant. The uterus itself will gain weight, owing to an increase in the size of its muscle fibers.

Throughout your pregnancy, if you put your hand over your uterus, you may feel the muscles tensing and releasing—contracting. The contractions, which are very slight and painless, occur about every 20 minutes. They help the uterus grow and ensure that blood gets to every part of it.

Later in your pregnancy your uterus may have grown enough to press on a large vein in your abdomen when you lie on your back. The pressure may make your blood pressure fall, and you may become dizzy or even faint. To avoid this, try to lie on your side instead of your back.

Your uterus may also begin to crowd your lungs during the middle to late part of your pregnancy. You may feel uncomfortable and need to take deep breaths to get enough air into your lungs. Sitting up straight—even when you are in bed—will help you breathe more easily.

Vagina
When you are pregnant, your vagina will become swollen, and the lining will thicken. The walls will become slicker and may produce some discharge. These changes will make the birth easier.

The secretions caused by the vaginal changes may make you feel sore or possibly cause unwanted odor. Call your doctor if this occurs. Don't douche to help control the problem. In fact, you should never douche during pregnancy. Wash the genital area often and dry carefully. Wear cotton underclothes for comfort.