Stratifying the analysis to look at the relationships specifically by race–ethnicity, the familiar pattern is seen in Table 4, although in the case of non-Hispanic black mothers, the increases in infant mortality are generally more pronounced. To better understand the possible effect of adherence to weight gain recommendations, we added adherence to the model, but given the relationship between length of gestation and weight gain, we limited the analysis to full-term (37–41 weeks of gestation) births in Table 4. Although the actual infant mortality rates were generally lower, the pattern of escalating risk across higher BMI categories remained the same for full-term births in the adjusted analysis without including gestational weight gain. Adding adherence to the weight gain recommendations had no effect on the pattern.
We also stratified the analysis by adherence to weight gain recommendations. With gaining the recommended weight as a reference, the adjusted ORs for the risk of an infant death for those gaining less than the recommended weight was 1.07 (95% CI 1.01–1.12); it was 1.04 (95% CI 0.99–1.09) for those gaining more than the recommended weight.
We did a series of subanalyses to assess whether they would have an effect on the relationships examined. Limiting the sample to women who did not have either diabetes or hypertension had no effect on the familiar pattern. Stratified analyses by payer source and parity also had no effect on the finding of increasing rates of infant death across prepregnancy BMI categories nor did stratifying by smoking status.
We used a data set of more than 6.4 million births to examine the relationship among prepregnancy BMI, gestational weight gain, and infant mortality. We found a strong relationship between prepregnancy BMI and infant mortality, which increased substantially across categories of obesity even after controlling for demographic and medical risk factors that have been shown to be related to infant mortality. These differences were consistent for both neonatal and postneonatal mortality and across the major causes of infant death. Many mothers whose gestational weight gain was within the ranges recommended by the IOM had births with slightly lower infant mortality rates than those who did not, but after adjustment, the differences largely disappeared.
These findings are generally consistent with past research although most prior studies focused on neonatal mortality and did not include either cause of death or weight gain data. Both a recent meta analysis10 and a large population-based study from Sweden8 found increasing rates of infant mortality across categories of BMI. Johansson et al found even larger increases in the adjusted ORs across BMI categories,8 although its applicability to a U.S. context may be limited. Sweden's rate of prepregnancy obesity is half that of the U.S. (9.9% compared with 20%) and its infant mortality rate is less than half that of the United States (2.6 compared with 6.0/1,000 in 2012).21 Chen et al, using data from the 1988 National Maternal and Infant Health Survey, found a clear relationship between prepregnancy BMI and neonatal death but a weak relationship with postneonatal death. They found a mixed effect of weight gain, with mothers in the underweight category who gained the least weight and the mothers in the obese category who gained the most weight, at greatest risk for infant death.4 Salihu et al,7 using Missouri data from 1978–1997 and limiting analysis to neonatal mortality, found a relationship between BMI and early neonatal but not late neonatal death for white mothers and a relationship with all neonatal death for black mothers.
Although this study relies on a large data set, it does have several limitations. Although the data account for 86% of all births in the United States, it does not include data from 12 states and thus is not fully representative of all U.S. births.14 As noted earlier, the information on prepregnancy weight as recorded on the birth certificate comes from measurement at the first visit or maternal recall. Birth certificate data do not permit stratification by these sources, so we were unable to examine the implications of the alternative bases for the measure of BMI. The measurement of BMI from the birth certificate is relatively new and there have been limited studies of the accuracy of birth certificate reporting of BMI. Park and colleagues22 found the BMI birth certificate measure to be generally valid and reliable for population-based research. Bodnar et al23 found BMI reporting most reliable for normal, overweight, and obese 2 and 3 categories but less accurate for underweight and obese 1 categories. Chen and colleagues4 also found a strong correlation between self-reported and measured prepregnancy BMI. A National Center for Health Statistics study of the quality of reporting on the revised 2003 birth certificate found reasonable data quality for the variables used here in our model such as gestational age, trimester prenatal care began, and source of payment; however, diabetes and hypertension appear to be underreported in birth certificate data.24 Like most published studies from the United States,4,25 we used the birth certificate measure of gestational weight gain. The limited studies on the accuracy of the birth certificate weight gain measure have found it to not be as reliable as the BMI measure,4 although Oken et al26 found a high correlation between self-reported and clinically measured weight gain.
Although the association between high BMI and infant death is robust, that is not proof that prepregnancy weight loss, whether by lifestyle changes or bariatric surgery, will change this relationship. It is possible that women who are predisposed to obesity have an underlying pathology that leads to the poor outcomes even if they lose weight. Studies of bariatric surgery suggest overall improved birth outcomes after weight loss,27 although these studies were not sufficiently powered to examine infant death.
We included small for gestational age in our analysis with little effect on the overall pattern of relationship between BMI and infant mortality. However, one exception was the case of underweight mothers who had a slightly higher risk of infant mortality in the initial model, but after the small-for-gestational-age variable was added to the final model, that relationship disappeared. The role of maternal underweight on infant outcomes, although not the focus of this study, deserves further attention.
This study, in conjunction with others that have demonstrated an association of maternal obesity with increased risk of perinatal death and infant mortality, builds a convincing foundation for future studies of perinatal outcomes for women who were obese and attained a normal or more normal BMI before conception. If such studies demonstrate that preconception weight loss is associated with improved outcome, even if the mechanism underlying that improvement is unknown, a public health campaign encouraging preconception weight loss to benefit not only the maternal but also the fetal and neonatal outcomes would be scientifically grounded, feasible, and compelling.
The nature of the weight gain recommendations once a woman is pregnant also deserves additional attention. The focus of the IOM guidelines was on general maternal and child health and not simply infant mortality, but the findings give pause concerning both the lack of adherence and the apparent limited benefits of compliance, at least in terms of infant mortality. The IOM's recommendation concerning more research on the effect of the subclasses of obesity seems particularly well-founded given these findings.
Concern about the effect of obesity on the nation's health has drawn the attention of providers, policymakers, and the public, most notably the First Lady's “Let's Move” campaign targeting childhood obesity.28 This study suggests the need for more research into the benefits of a systemwide effort to prevent obesity among women of reproductive age.
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