This is a population-based retrospective cohort study of all live born singleton, full-term infants born to Missouri residents between January 1, 2000 and December 31, 2006 (N=502,452). Data was obtained from Missouri vital records, which includes birth certificate records linked to hospital discharge information, for the available period of 2000–2006. Women with pre-pregnancy BMI ≥30 kg/m2 were included. Exclusion criteria were: 1) fetuses with major congenital anomalies (n= 872, 1.3%), and 2) women with diabetes (n= 5,830, 8.3%) or chronic hypertension (n = 1,773, 2.7%) as documented in the birth certificate or hospital discharge data. Women with either pregestational or gestational diabetes were excluded due to the inability to reliably classify the type of diabetes based on the birth certificate or ICD-9 coding. Inclusion was limited to term infants to avoid confounding of neonatal outcomes due to complications associated with prematurity.
The primary predictor of interest was maternal BMI. BMI was calculated by self-reported pre-pregnancy weight in kilograms divided by height in meters squared. The World Health Organization (WHO) separates obesity (BMI ≥ 30 kg/m2
) into three classes – class I (30–34.9 kg/m2
), class II (35.0–39.9 kg/m2
) and class III (≥ 40 kg/m2
As the objective of this study was to determine the impact of super obesity on perinatal outcomes and whether there was a dose-response to increasing obesity, we combined class I and II as obese (30–39.9 kg/m2
), and separated class III into morbid obesity (40–49.9 kg/m2
) and super-obesity, defined as BMI ≥ 50 kg/m2
The primary outcomes of interest were preeclampsia, method of delivery, macrosomia (birth weight >4500 grams), and composite neonatal morbidity, which included low Apgar score (<7 at 5 minutes), birth trauma, neonatal infection, neonatal hypoglycemia, respiratory distress syndrome, neonatal seizures, neonatal length of stay > 5 days, and/or meconium aspiration syndrome. Low birth weight was defined as <2500 grams. If a diagnosis such as preeclampsia, birth trauma, or respiratory distress syndrome was documented in either the birth certificate or the hospital discharge data, then the condition was considered present. Use of the combined birth certificate and hospital discharge data has been found to be more accurate for perinatal outcomes compared to birth certificate data alone.9, 10
Various maternal socio-demographic characteristics have been shown to be associated with maternal obesity and were evaluated as potential confounders in this study. Maternal education was categorized as high, average, or low based on age and years of education.11
Corrected for maternal age, average education included women within 2 grades of their expected level, and low education was 2 or more grades below expected grade. Greater than 12 years of education was considered high, regardless of maternal age. The R-GINDEX was used to categorize prenatal care as no care, inadequate, adequate, intermediate, intensive, or missing based on initiation of prenatal care, total number of visits, and gestational age at delivery.12,13
Smoking status was determined by maternal self report on birth certificate records.
Bivariate analyses were completed using the chi-square (χ2) , Fisher exact test, and t-test, as appropriate. Outcomes were assessed using Cochrane-Armitage test for linear trend and multivariable regression for adjusted risk. Multivariable logistic regression models were used to evaluate outcomes, controlling for maternal age, race, parity, smoking status, marital status, Medicaid use, prenatal care, level of education, primary scheduled cesarean and repeat cesarean. Mode of delivery was categorized by birth certificate designation as vaginal, operative vaginal, vaginal birth after cesarean (VBAC), primary emergent cesarean, primary elective cesarean, and repeat cesarean. For clarity, primary elective cesarean is referred to as primary scheduled cesarean. Comparisons were made among BMI groups (obese, morbidly-obese, and super-obese). Adjusted relative risk (aRR) and 95% confidence interval (CI) were calculated. A value of P<0.05 on two-tailed tests was considered significant.
All analyses were completed using SAS version 9.2 (SAS Institute Inc., Cary, NC). Approval for human subject research and a waiver of informed consent were received from the Institutional Review Board at Saint Louis University and the Missouri Department of Health and Senior Services, Section for Epidemiology for Public Health Practice.