The purpose of the study is to determine if a low glycemic load diet reduces the gain of body fat and insulin resistance during the last half of pregnancy in obese women.

Obese women will be recruited to participate in the study between 14-18 weeks gestation. After determining usual dietary intakes, visceral fat thickness, and the metabolic response to a 100g oral glucose tolerance test (OGTT) at 20 weeks gestation, 45 of the women will be randomly assigned to a low glycemic load (GL) diet and 65 women to a regular low fat diet (current standard care). They will follow their assigned diet for the last 20 weeks of gestation. A dietitian will advise the women and will monitor them throughout the study. At 28 and 34 weeks gestation, the women will come to the General Clinical Research Center (GCRC) at San Francisco General Hospital in the fasting state and the hormonal and metabolic response will be measured to another 100g oral glucose tolerance test (OGTT), and total body fat will be measured. Additional measurements include: 1) longitudinal measurements of maternal visceral fat thickness and fetal size and adiposity at 20 and 34 weeks gestation by ultrasound, 2) collection of cord blood and placental tissue, and 3) measurement of neonatal body composition by dual-energy x-ray absorptiometry and anthropometry at 2-3 weeks of age. Birth outcome data for mother and infant will be collected. To determine the metabolic response to the study diet, 25 women from each of the two diet groups will be assigned to a metabolic subsample for measuring rates of hepatic glucose production and lipolysis at 34 weeks.

Inclusion Criteria:

• >35% body fat
• 18 to 40 years of age
• speak English or Spanish

Exclusion Criteria:

• have diabetes prior to pregnancy
• diagnosed with GDM in current pregnancy
• smoke
• abuse alcohol or drugs
• have a metabolic disease