In an upcoming issue of a scientific journal, Seminars in Perinatology, @UofR baby doc Loralei L. Thornburg says that, while it may seem counterintuitive, obese women may lack many vitamins necessary to have a healthy pregnancy. In particular, folic acid is a necessary component both of successful implantation of a fertilized egg as well as preventing certain heart and spine defects in newborns, is often found to be lacking in overweight women.
Additionally, overweight women are at an increased risk of sleep apnea, a condition where the sufferer experiences abnormal pauses in breathing while sleeping. This is true in all cases, but many women experience sleep apnea during pregnancy because they are forced to sleep on their backs towards the end of the pregnancy.
For more information on obesity and pregnancy, see the following U of R article: