The purpose of this study is to evaluate the flow of oxygen and blood necessary for nutrient transfer across the placenta during the second and third trimesters in pregnancy in women who are obese before pregnancy compared to women who have an average weight before pregnancy. This study will evaluate blood flow across the placenta by functional Magnetic Resonance Imaging (fMRI) using arterial spin labeling (ASL). The investigators hypothesize that there will be differences in placental oxygenation and blood flow among women with obesity as compared to those with normal weight. Participants will be asked to complete Magnetic Resonance Imaging scans (MRIs), the first in the second trimester at 20-24 weeks and the second in the third trimester at 30-34 weeks.