In every issue of WebMD the Magazine , we ask experts to answer readers' questions about a wide range of topics, including some of the oldest -- and most beloved -- medical myths out there. In our July-August 2010 issue, we asked Julie Redfern, RD, LDN, a registered dietitian at Brigham and Women's Hospital in Boston, to weigh in on just how much a pregnant woman should eat.

Q: Now that I'm pregnant, I get to "eat for two," right?

A: While there may be days when you feel so hungry you could eat a horse, the idea that you should "eat for two" is, alas, FALSE.

"That is an old myth," says Julie Redfern, RD, LDN, a registered dietitian at Brigham and Women's Hospital in Boston. "Many women who do 'eat for two' end up gaining an excessive amount of weight."

You certainly should be increasing the amount of certain nutrients. But you need only about 300 more calories per day, Redfern says, about the equivalent of "one slice of whole grain bread and one tablespoon of peanut butter." If you put on too much weight during pregnancy, you increase your risk of gestational diabetes, backaches, high blood pressure, and needing a cesarean birth because your baby is very large.

Conversely, gaining too little weight during pregnancy is a concern. It can lead to low birth weight, premature delivery, and, later, developmental delays, learning disabilities, and chronic health problems in your baby.