Theresa Sprague weighed 225 pounds when she conceived twins. Though her size put her at a higher risk for serious complications, luckily she didn't end up having health problems. But she did have a lot of anxiety: Sprague wasn't able to hear her babies' heartbeats until she was halfway through her pregnancy -- not at the usual 10 to 12 weeks -- because sound waves don't travel well through fatty tissue. An ultrasound helped ease her worries a little, but her extra body mass also made it difficult to get a good image of the babies. When Sprague delivered her sons at 38 weeks gestation, the mom from Maryland, New York, breathed a big sigh of relief.

More than half of women ages 20 to 44 are overweight, and of those, nearly a third are obese, according to the Centers for Disease Control and Prevention. Overweight women are already more likely to have health complications -- but pregnancy increases the risks for both mother and baby. "Obesity is a disease, and as with many diseases, there's a greater chance of problems during pregnancy," says Vivian Dickerson, MD, director of women's healthcare and programs at Hoag Memorial Hospital, in Newport Beach, California.

Doctors aren't as concerned about what your scale says as they are about your body mass index (BMI), an estimate of body fat based on weight and height (calculate yours at parents.com/BMI). But a high BMI doesn't necessarily mean you're going to have medical issues: Many plus-size moms have problem-free pregnancies. Weight is a number just like maternal age. "Being older than 35 places some women at higher risk for problems, but most do just fine," says Dr. Dickerson. The same holds true for overweight women. Knowing what to watch out for if you're plus-size and pregnant is the first step to having a healthy nine months.

Finding a doctor who is "size friendly" can be a challenge, says Bruce D. Rodgers, MD, coauthor of Your Plus-Size Pregnancy and director of maternal-fetal medicine at Women & Children's Hospital of Buffalo. "Although your doctor will probably want to discuss medical issues regarding your weight, if he (or the office staff) makes you feel guilty about your size or discourages you from trying to conceive, it's time to find a more sensitive, educated healthcare provider."

Gestational diabetes, a temporary form of high blood sugar during pregnancy, affects up to 15 percent of obese women -- triple the rate of average-size women. Uncontrolled, it increases the risk of fetal birth defects and congenital heart problems. Babies born to moms with gestational diabetes tend to be large, which can pose problems in the delivery room. "Big babies can mean bigger episiotomies, vaginal lacerations, and an increased chance of a cesarean section," says Dr. Rodgers. Typically, expectant moms are tested for diabetes toward the end of their second trimester, but because extra weight also increases your risk of type 2 diabetes, you should be tested within the first six weeks after delivery and yearly after that. If you're diagnosed with diabetes (gestational or type 2), you may need to take insulin medication and will definitely need to make healthy food choices to keep the condition in check.

Eight percent of all pregnant women develop preeclampsia, a form of high blood pressure that usually occurs after the 20th week of pregnancy (though most cases are mild and occur at the end of pregnancy). Your odds quadruple if you're obese. The disease increases urine protein levels and causes fluid retention in the hands and feet. Bed rest and medications usually help, but if the condition isn't treated properly, it can be life-threatening for both mom and baby.

"Being in labor can be as physically demanding as running a marathon," Dr. Dickerson says. That's why women who are overweight and less physically fit tend to experience longer, harder labors and have higher rates of c-sections. Postdelivery recovery also can be more taxing. "Overweight women are at greater risk of infection following a cesarean because their stomachs hang over the incision site, keeping it moist and prone to bacteria, and fat itself is more prone to infection," says Cornelia van der Ziel, MD, coauthor of Big, Beautiful, and Pregnant. For this reason, some doctors perform vertical incisions on obese women instead of the lower horizontal or bikini cut.