Pregnancy
During pregnancy, a woman's body undergoes dramatic changes that are normal and necessary for a healthy baby. By the end of a normal pregnancy, a woman may gain an average of about 25 pounds. The baby accounts for only one third of that weight.
In addition to the growing baby, weight gain during pregnancy is due to changes in the placenta, amniotic fluid, uterine and breast tissue, and maternal fat. Her blood volume also increases by as much as 40 percent. The extent of these changes is markedly different from mother to mother which is why weight gain differs so much.
It is widely accepted that the fat gained during pregnancy is needed to produce milk once the baby is born. The baby also gains fat as pregnancy proceeds, especially in the last three months of the pregnancy.