Journal of Perinatology (2016) 36, 1055–1060 © 2016 Nature America, Inc., part of Springer Nature. All rights reserved 0743-8346/16 www.nature.com/jp ORIGINAL ARTICLE Gestational weight gain and preterm birth: disparities in adolescent pregnancies CL Woolfolk1, LM Harper2, L Flick1, K Mathews3 and JJ Chang1 OBJECTIVE: To examine racial differences in the association between gestational weight gain and preterm birth subtypes among adolescents. STUDY DESIGN: We conducted a retrospective cohort study of 211 403 adolescents using 2012 United States natality data. The outcome was preterm birth and the primary exposure was gestational weight gain. Multinomial logistic regression analyses were used to estimate adjusted odds ratios, stratified by race and body mass index (BMI). RESULTS: Black and White mothers who gained below the recommendations had increased risks for spontaneous preterm birth in all BMI categories, except obese. All Hispanic mothers who gained below the recommendations had increased risks of spontaneous preterm birth. White normal and overweight mothers and Black, Hispanic and Other normal weight mothers who exceeded the recommendations had decreased risks of spontaneous preterm birth. CONCLUSION: The effect of gestational weight gain on spontaneous and medically indicated preterm birth is modified by race and BMI. Journal of Perinatology (2016) 36, 1055–1060; doi:10.1038/jp.2016.149; published online 8 September 2016 INTRODUCTION effect modifier in the relationship between gestational weight Preterm birth (PTB), the birth of an infant before 37 weeks gain and PTB in adolescent mothers, these studies use different gestation, is a major contributor to infant morbidity and mortality. measures of gestational weight gain and reference groups, have In 2014, 1 out of 10 infants were born preterm in the United limited generalizability because they usually take place in one 11 States.1 PTB rates among African American and American Indian/ state, and fail to examine PTB by subtype. The objective of this Alaskan Native women are significantly higher compared with study was to examine racial and BMI differences in the association White women, while Asian/Pacific Islanders have significantly between gestational weight gain and spontaneous and medically lower PTB rates.2–4 Inadequate gestational weight gain, maternal indicated PTB among adolescents, using the current IOM pre-pregnancy weight and race have been identified as risk factors guidelines. for PTB.5–9 There are two subtypes of PTB, spontaneous and medically indicated, which are thought to result from different causes.6 Adolescent pregnancy is characterized by increased risks MATERIALS AND METHODS – of many adverse birth outcomes, including PTB.10 12 Because of We conducted a population-based retrospective cohort of 2012 United the additional risks associated with adolescent pregnancy, it is States (US) natality data, which includes birth certificate data of all live important to study the impact of these factors on PTB subtypes births delivered in the US. The 2012 natality data, obtained from the among adolescent mothers. National Center for Health Statistics, includes all registered births occurring fi in the US. The study used de-identified data and was exempt from Gestational weight gain is an important modi able factor that o affects PTB.6,13 In 2009, the Institute of Medicine (IOM) revised the institutional board review. Our study included all nulliparous women 20 years old, who delivered live, singleton infants 20 to 44 weeks gestation in gestational weight gain guidelines that were designed to reduce 2012. We excluded multiple gestations, congenital anomalies and subjects infant morbidity and mortality. The IOM guidelines, based on pre- with missing data on BMI. pregnancy body mass index (BMI), recommend ideal gestational Maternal BMI, calculated as weight per height (kg m − 2), was weight gain ranges to improve birth outcomes. The current categorized as underweight (BMI o18.5 kg m − 2), normal weight guidelines are based on adult BMI categories, however, adult BMI (BMI 18.5-24.9 kg m − 2), overweight (BMI 25.0-29.9 kg m − 2) and obese − categories are not always applicable to adolescents. The IOM (BMI ≥ 30 kg m 2) based on self-reported pre-pregnancy weight and recommends that adolescents use adult BMI categories until height. The self-reported maternal races were categorized as: White, there is more research to determine if separate categories are non-Hispanic; Black, non-Hispanic; Hispanic; and Other, non-Hispanic. The 14 Other, non-Hispanic maternal race category includes American Indian/ necessary. Alaskan Native, Asian, and Pacific Islander. Given the racial disparities in PTB and the increased risk of PTB The primary exposure was gestational weight gain, which was obtained among adolescents, it is important to examine whether maternal from the birth certificate. The IOM recommendations were used to classify race and BMI modify the relationship between gestational weight gestational weight gain into three categories: below, met, and exceeded gain and PTB. Although a few studies have evaluated BMI as an IOM recommendations. The IOM recommends that underweight women 1Department of Epidemiology, Saint Louis University College for Public Health and Social Justice, St. Louis, MO, USA; 2Division of Maternal Fetal Medicine, Department of Obstetrics and Gynecology. University of Alabama at Birmingham, Birmingham, AL, USA and 3Department of Obstetrics, Gynecology and Women’s Health, School of Medicine, Saint Louis University in St. Louis, St. Louis, MO, USA. Correspondence: Dr JJ Chang, Department of Epidemiology, Saint Louis University College for Public Health and Social Justice, 3545 Lafayette Avenue, Room 476, St. Louis, MO 63104, USA. E-mail: [email protected] Received 15 December 2015; revised 4 July 2016; accepted 11 July 2016; published online 8 September 2016 Gestational weight gain and preterm birth CL Woolfolk et al 1056 gain 1 to 1.3 pounds per week, normal weight women gain 0.8 to 1 and the Kotelchuck index, the odds of spontaneous and medically pounds, overweight women gain 0.5 to 0.7 pounds, and obese women indicated PTB among underweight adolescents were significantly gain 0.4 to 0.6 pounds during the second and third trimesters. A weight increased among those below the IOM recommendations, gain of 1.1 to 4.4 pounds in the first trimester is assumed for each BMI.14 To compared with those who met the IOM recommendations (aOR determine the gestational weight gain, we calculated the normal weight gain range for each week using the recommended weekly weight gain for 1.87, 95% CI: 1.13, 3.07 for medically indicated and aOR 1.52, 95% each BMI. The number of gestational weeks after the first trimester for each CI: 1.25, 1.85 for spontaneous PTB). Among normal weight subject was calculated by subtracting 13 from the gestational age. adolescents, the odds of spontaneous PTB were significantly The lower limit of the normal weight gain range was calculated by adding increased among those below the IOM recommendations (aOR the lower limit of the assumed first trimester weight gain to the product fi 1.50, 95% CI: 1.33, 1.68 for spontaneous PTB); the odds of of the number of gestational weeks after the rst trimester and the lower fi limit of the recommended weight gain for that BMI. The upper limit of spontaneous PTB were signi cantly decreased among those who the normal weight gain range was calculated by adding the upper limit exceeded the IOM recommendations (aOR 0.81, 95% CI: 0.74, of the assumed first trimester weight gain to the product of the number of 0.89). Among overweight adolescents, the odds of spontaneous gestational weeks after the first trimester and the upper limit of the PTB were significantly increased among those below the IOM recommended weight gain for that BMI. Each subject’s total gestational recommendations (aOR 1.31, 95% CI: 1.03, 1.67); the odds of weight gain was then compared to the normal weight gain range for their spontaneous PTB were significantly decreased among those who BMI and gestational age. Gestational weight gain within the normal weight gain range was categorized as ‘met IOM guidelines,’ less than the normal exceeded the IOM recommendations (aOR 0.70, 95% CI: 0.58, weight gain range was categorized as ‘below IOM guidelines,’ and above 0.85). Among obese adolescents, the odds of spontaneous PTB the normal weight gain range was categorized as ‘exceeded IOM were significantly decreased among those below the IOM recommendations.’ recommendations (aOR 0.77, 95% CI: 0.61, 0.96). The outcome of interest was PTB, which included births before 37 weeks gestation. PTB was calculated using the clinical estimate of gestational age. PTB was further categorized into spontaneous and medically indicated Black maternal race PTB. Medically indicated PTB consists of all PTB where a maternal medical aORs for spontaneous and medically indicated PTB among those condition or labor induction was present. Maternal medical conditions reporting Black maternal race are presented in Table 3. After include: pre-pregnancy diabetes, gestational diabetes, pre-pregnancy adjusting for confounders, the odds of spontaneous and medically hypertension, gestational hypertension and eclampsia. Spontaneous PTB fi consists of all PTB without labor induction and maternal medical indicated PTB among underweight adolescents were signi cantly conditions (diabetes, hypertension or eclampsia).15 increased among those below the IOM recommendations (aOR The Pearson Χ2 test was used to assess differences in sample 2.47, 95% CI: 1.18, 5.15 for medically indicated and aOR 1.50, 95% characteristics by gestational weight gain. Effect modification by maternal CI: 1.12, 1.99 for spontaneous PTB). Among normal weight race and BMI was assessed through logistic regression using a three-way adolescents, the odds of spontaneous and medically indicated interaction term between race, BMI and gestational weight gain. A P-value PTB were significantly increased among those below the IOM fi Χ2 of 0.05 was used to assess the signi cance of the Wald statistic.