(CNN) - Many expectant mothers are trying to keep the weight off. But experts are saying that's not a healthy habit.
Gynecologists say some women are so obsessed with staying thin that they try to keep the weight off even while they're pregnant. This can cause premature delivery and pre-term labor.
"Both of those problems can be very serious, can cause very serious birth defects for the baby and can also cause problems with mom," said Dr. Angela Marshall with Comprehensive Women's Health.
Healthy weight gain during pregnancy depends on the woman.
Women who are under weight should gain between 28 and 40 pounds. Those with normal weight should gain between 25 and 35 pounds, and for women who are obese, approximately 11 to 20 pounds.
That can be a dilemma for women who are overweight, because the more they gain, the more they're at risk for developing conditions like high blood pressure and gestational diabetes.
If the women gain weight in a healthy manner, it usually goes away.
"Much of the weight that they gain during their pregnancy is actually lost during the birthing process," Marshall said.