Doctors now recommend that most women gain between 25 and 35 pounds during pregnancy, depending on their size and weight. If your weight gain stays within this range, your baby will get enough nutrients without becoming too big to deliver vaginally. You'll also find it easier to return to your pre-pregnancy weight—especially if you're nursing.

Who Should Gain Less, Who Should Gain More

Women who are underweight for their height are advised to spend the first trimester catching up, and the last two trimesters putting on 25 to 35 pounds. Those who start out 10 to 20 percent overweight, on the other hand, should not diet while they are pregnant. The growing fetus can't get the nutrients it needs from its mother's fat stores. Doctors recommend a minimum gain of 15 pounds—from high-quality foods—to safeguard the baby's health.

The recommended weight gain for mothers carrying twins doesn't double, but it's 10 pounds higher—from 35 to 45 pounds. It's even higher for triplets.

Monitoring Your Weight Gain

During the first trimester, you should put on about three or four pounds. The biggest increase will be during the second trimester, when gaining a pound or more a week is normal. You'll continue to add a pound a week until the ninth month, when half a pound, or even none at all, is more typical. Some women start off with a bang and then taper off. Others don't gain much at first because of morning sickness, start gaining in the second trimester, and catch up by the third.

When to Check with Your Doctor

Check with your doctor if you experience any of the following:

• You fail to gain weight for two weeks or more between your fourth and eighth months.
• You gain more than three pounds a week during the second trimester, or more than two pounds a week during the third trimester. This is a possible sign of toxemia.
• You gain most of your weight in any one trimester. You won't be advised to diet, but you will be given guidelines on how to eat more nutritiously for the remaining months.