Obesity has become so pervasive that it is now considered a major health concern during pregnancy. Almost 56% of women aged 20-39 years in the United States are overweight or obese, based on the World Health Organization’s criteria for body mass index (BMI) and data from the 2009-2010 National Health and Nutrition Examination Survey (NHANES). Moreover, 7.5% of women in this age group are morbidly obese, with a body mass index (BMI) greater than 40 kg/m2 (JAMA 2012;307:491-7).
Obesity in pregnancy not only increases the risk of spontaneous abortions and congenital anomalies, it also increases the risk of gestational diabetes (GDM), hypertensive disorders, and other metabolic complications that affect both the mother and fetus.
Dr. Patrick Catalano
Of much concern is the increased risk of fetal overgrowth and long-term health consequences for children of obese mothers. Obesity in early pregnancy has been shown to more than double the risk of obesity in the offspring, which in turn puts these children at risk for developing the metabolic syndrome – and, as Dr. Thomas Moore pointed out in September’s Master Class – appears to program these offspring for downstream cardiovascular risk in adulthood.
Mean term birth weights have risen in the United States during the past several decades. In Cleveland, we have seen a significant 116 g increase in mean term birth weight since 1975; this increase encompasses weights from the 5th to the 95th percentiles. Even more concerning is our finding that the ponderal index in our neonatal population has increased because of decreased fetal length over the last decade.
Some recent studies have suggested that the increase in birth weight in the United States has reached a plateau, but our analyses of national trends suggest that such change is secondary to factors such as earlier gestational age of delivery. Concurrently, an alarming number of children and adolescents – 17% of those aged 2-19 years, according to the 2009-2010 NHANES data – are overweight or obese (JAMA 2012;307:483-90).
How to best treat obesity for improved maternal and fetal health has thus become a focus of research. Studies on lifestyle interventions for obese women during pregnancy have aimed to prevent excessive gestational weight gain and decrease adverse perinatal outcomes – mainly macrosomia, GDM, and hypertensive disorders.
However, the results of this recent body of research have been disappointing. Lifestyle interventions initiated during pregnancy have had only limited success in improving perinatal outcomes. The research tells us that while we may be able to reduce excessive gestational weight gain, it is unlikely that we will be successful in reducing fetal overgrowth, GDM, or preeclampsia in obese women.
Moreover, other studies show that it is a high pregravid BMI – not excessive gestational weight gain or the development of GDM – that plays the biggest role in fetal overgrowth and fetal adiposity.
A paradigm shift is in order. We must think about lifestyle intervention and weight loss before pregnancy, when the woman’s metabolic condition can be improved in time to minimize adverse perinatal metabolic outcomes and to maximize metabolic benefits relating to fetal body composition and metabolism.
Role of prepregnancy BMI
In 2008, the Institute of Medicine (IOM) and National Research Council reexamined 1990 guidelines for gestational weight gain. They concluded that excessive weight gain in pregnancy was a primary contributor to the development of obesity in women. In fact, according to the 2009 IOM report, “Weight Gain During Pregnancy: Reexamining the Guidelines” (Washington: National Academy Press, 2009), 38% of normal weight, 63% of overweight, and 46% of obese women had gained weight in excess of the earlier guidelines.
Helping our patients to gain within the guidelines is important. Excessive gestational weight gain is a primary risk factor for maternal postpartum weight retention, which increases the risk for maternal obesity in a subsequent pregnancy. It also has been associated with a modest increased risk of preterm birth and development of type 2 diabetes.
Interestingly, however, high gestational weight gain has not been related to an increased risk of fetal overgrowth or macrosomia in many obese women. Increased gestational weight gain is a greater risk for fetal overgrowth in women who are of normal weight prior to pregnancy (J. Clin. Endocrinol. Metab. 2012;97:3648-54).
Our research has found that in overweight and obese women, it is maternal pregravid BMI – and not gestational weight gain – that presents the greatest risk for fetal macrosomia, and more specifically, the greatest risk for fetal obesity. Even when glucose tolerance levels are normal, overweight and obese women have neonates who are heavier and who have significant increases in the percentage of body fat and fat mass (Am. J. Obstet. Gynecol. 2006;195:1100-3).