The Combined Effect of Maternal Prepregnacy Body Mass Index and Weight Gain During Pregnancy On Infant Macrosomia

Introduction: Macrosomia (infant birth weight more than 4000 grams) is a condition with high prevalence in the United States (7.6%) and etiology not sufficiently understood, that is associated with traumatic births and adverse health outcomes (such as obesity and diabetes in childhood and later in life). Excessive weight gain during pregnancy and high prepregnancy maternal body mass index (BMI) have been linked to infant high birth weight, but examination of their joint effect on macrosomia may advance etiological understanding. 

Methods: We analyzed data from 98,141 mothers and singletons from South Carolina Department of Health's 2004-2005 birth certificates. Based on their BMI, women were classified as underweight (<19.8), normal weight (19.8-26.0), overweight (26.1-28.9), obese (29.0-34.9) or very obese (>35.0) and the average weight gain per week was defined as very low (<0.25 lbs/week), low (0.25-0.49 lbs/week), moderate (0.50-1.50 lbs/week), high (1.51-1.75 lbs/week) and very high (>1.75 lbs/week). Multiple logistic regression models were used to test the associations among combinations of maternal pre-pregnancy BMI and average gestational weight gain per week with the development of macrosomia.

Results: The risk of macrosomia increases mainly with gestational weight gain, although prepregnancy BMI had an important contribution, particularly for obese and very obese mothers compared to normal weight women who gained moderate gestational weight (OR ranges from 1.2 to 3.1 for obese and from 1.9 to 5.6 for very obese women with increasing average gestational weight gain/week from very low to very high). For overweight women the risk increases with weekly average gestational weight gain from moderate to very high (OR 1.6 to 3.3), while underweight and normal weight women have an increased risk of infant macrosomia only if they gain very high average gestational weight/week (OR=1.8 respectively 2.6).

Conclusions: The risk of infant macrosomia is associated with very high maternal average gestational weight gain/week in the last two trimesters of pregnancy, regardless of maternal prepregnancy BMI. However, obese and very obese mothers are at risk for macrosomic infants, regardless of their weekly gestational weight gain. While normal weight and overweight women could lower the risk of macrosomia by gaining low and very low weekly gestational weight, for obese and very obese women preconception advice to achieve healthier weight would be more effective than weight gain restriction during pregnancy.