Study examines link between genetically elevated maternal BMI and higher offspring birth weight

In a study that included more than 30,000 women, genetically elevated maternal body mass index (BMI) and blood glucose levels were potentially causally associated with higher offspring birth weight, while genetically elevated maternal systolic blood pressure was potentially causally related to lower birth weight, according to a study appearing in JAMA.

Neonates born to overweight or obese women are more likely to be large for gestational age. The precise mechanisms underlying this association are poorly understood. It is important to understand which maternal traits are causally associated with birth weight because this may facilitate targeted development of interventions to be tested and enable evidence-based recommendations for pregnant women.

Rachel M. Freathy, Ph.D., of the University of Exeter, and Debbie A. Lawlor, Ph.D., of the University of Bristol, U.K., and colleagues tested for genetic evidence of causal associations of maternal BMI and related traits with birth weight. Data from 30,487 women in 18 studies were analyzed. Participants were of European ancestry from population- or community-based studies in Europe, North America, or Australia and were part of the Early Growth Genetics Consortium. Live, term offspring born between 1929 and 2013 were included.

The researchers found that a genetically higher maternal BMI of 4 points was associated with a 1.9 ounces higher offspring birth weight. In addition, a genetically higher circulating maternal fasting glucose of 7.2 mg/dL was associated with a 4 ounces higher birth weight, whereas genetically higher maternal systolic blood pressure (SBP) of 10 mm Hg was associated with a 7.3 ounces lower birth weight.

"These results provide evidence that genetically elevated maternal glucose and SBP may have directionally opposite causal associations with birth weight. The estimated associations between these maternal traits and birth weight (either increased or reduced) are substantial and of clinical importance. They support efforts to maintain healthy gestational glucose and blood pressure levels to ensure healthy fetal growth," the authors write.

"If replicated, these findings may have implications for counseling and managing pregnancies to avoid adverse weight-related birth outcomes."