Maternal obesity, high gestational weight gain and diabetes mellitus during pregnancy are known risk factors that correlate with high infant birth weight and the mother's race. Previous studies have focused on low birth weight, prematurity and infant mortality. This study examined the interaction between race, maternal risk factors and high infant birth weights at the population level in South Dakota to identify factors contributing to the high Native American infant birth weights. We hypothesized that high infant birth weights were associated with maternal diabetes, obesity and high gestational weight gain, and that Native American infants' higher birth weights were related to the prevalence of diabetes and obesity.
De-identified birth certificate data was provided by the South Dakota Department of Health. We used data for live infant births to South Dakota resident mothers from 2006 through 2011. The mothers were categorized as Native American or white by the mother's self-reported primary race. Infants were excluded from the study population for missing data, birth weight less than 350 g or gestational age less than 24 weeks or greater than 45 weeks. The study population included 11,416 Native American infants and 59,263 white infants for a total study population of 70,679 infants. Maternal variables (race, pre-pregnancy weight and body mass index [BMI], gestational weight gain, pre-pregnancy diabetes mellitus [DM], gestational diabetes [GDM] and delivery BMI) and infant variables (gestational age and birth weight) were analyzed using SPSS software.
The mean birth weight (BW) of Native American (NA) infants (3377 g) was significantly greater than the mean BW of white (W) infants (3315 g) even though NA infants had a younger mean gestational age (p = 0.006). More NA infants were categorized as high birth weight (HBW) (11.8 percent) than W infants (8.5 percent). Both DM and GDM were significantly more common among NA mothers. Infants of NA mothers with GDM had a higher mean BW than infants of W mothers with GDM. There were more overweight and obese NA mothers (p = 0.006). In each maternal BMI category, NA infants had a higher mean BW. Mean BW was even higher for infants born to mothers with excessive gestational weight gain (GWG) for their BMI. The infants with the highest mean BW were born to obese NA mothers with GDM and excessive GWG (3680 g). Multivariable linear regression showed that race was the most significant variable affecting infant BW (R2 = 0.57, F = 692). Pre-pregnancy BMI, GWG and excessive GWG were also significant. The most significant interaction variables were race and GDM and race and BMI.
Native American race, gestational diabetes mellitus, overweight and obese BMI, and excessive gestational weight gain for BMI were the most significant maternal factors associated with high infant birth weight. Mothers with any one risk factor gave birth to heavier infants. Mothers with all risk factors had infants with the highest mean birth weights in South Dakota. This large population-based study provides evidence that Native American mothers in South Dakota with GDM, overweight or obese BMI and excessive GWG are more likely to give birth to high birth weight infants. At-risk mothers should be educated regarding the risks and potential complications of high birth weight infants.