According to WebMD, the average woman should gain between 2 to 4 pounds during her first trimester of pregnancy. After the first three months of pregnancy, the woman should expect to gain approximately 1 pound per week until she has the baby.
WebMD explains that an average-sized woman should gain anywhere from 25 to 35 pounds total during her pregnancy. An underweight woman should expect to gain anywhere from 28 to 40 pounds during her pregnancy, while an overweight woman may only need 15 to 25 extra pounds. The extra body weight a woman gains is caused by the baby, the extra blood flow to the baby, the placenta, the amniotic fluid, extra breast tissue, an enlarged uterus and fat storage for delivery, according to WebMD.