One of the most striking changes during pregnancy is a gain in weight. Twenty-four pounds is the average additional weight that the mother-to-be puts on. Of this amount, only a small fraction, about two pounds, should be gained in the first three months; about eleven pounds are gained in each of the other three-month periods.

This gain in weight is accounted for not only by the child you are carrying but by other factors as well. The average weight of the infant at birth is 7 1/2 pounds; the placenta (the organ which provides oxygen and food essential for the life of the fetus and for the elimination of its waste products) weighs 1 pound; and the fluid in the sac in which the baby lives is 1 1/2 pounds. At the end of pregnancy the uterus weighs about 2 1/2 pounds and the breasts 3 pounds.

Most expectant mothers are afraid that childbearing will leave them with an ungainly figure and that they will develop a stomach that will stick out even after the birth of the baby, but there is no reason why they should have this fear. The extreme prominence of the lower abdomen is most often caused by weakness of the abdominal muscles, a weakness that happily can be substantially overcome by proper exercise. An additional benefit, in the eyes of some physicians, is that by strengthening the muscles of the abdomen and increasing their elasticity one makes the actual birth much easier.

Abdominal streaking, or stretch marks on the stomach, is a situation which occurs frequently because of the stretching of the skin over the abdomen during the last months of the pregnancy. Another readily apparent change is that your breasts will become larger, firmer, and more tender. Because there is an increased blood supply to the breasts as pregnancy advances, the nipples and the elevated pigmented area immediately around them become darker in color. These breast changes are, of course, in preparation for the nursing of the baby. The veins beneath the skin throughout your body may become more prominent because of the increased blood supply. With some women this condition is particularly noticeable in the legs and can lead to permanent varicose veins or broken capillaries. Proper exercise, especially of the legs, may reduce the likelihood of this happening.

Another change affects the coloring of your skin. Pigment, deposited on the forehead, cheeks, and nose, gives the expectant mother a rosy, healthy appearance. About two-thirds of pregnant women have still another, change in their skin texture and coloration. Sometimes, early in pregnancy, small red elevations will appear over the face, neck, and arms. These skin changes disappear following delivery and will have no lasting effect on your complexion.

The uterus undergoes a remarkable change during pregnancy. It is converted from an almost solid organ about the size of a woman's closed fist into a large, thin-walled, muscular sac capable of containing the baby, the placenta, and a large quantity of fluid as well.