Prepregnancy body mass index and risk of preterm birth: association heterogeneity by preterm subgroups.
BACKGROUND
To evaluate the association between prepregnancy body mass index (BMI) is associated with early vs. late and medically-induced vs. spontaneous preterm birth (PTB) subtypes.


METHODS
Using data from the Boston Birth Cohort, we examined associations of prepregnancy BMI with 189 early (<34 completed weeks) and 277 late (34-36 completed weeks) medically-induced PTBs and 320 early and 610 late spontaneous PTBs vs. 3281 term births (37-44 weeks) in multinomial regression. To assess for mediation by important pregnancy complications, we performed sequential models with and without hypertensive disorders of pregnancy, chorioamnionitis, and gestational diabetes.


RESULTS
Prevalence of prepregnancy obesity (BMI ≥ 30.0 kg/m2) was 28% among mothers with medically-induced PTBs, 18% among mothers with spontaneous PTBs, and 18% among mothers with term births (p = <0.001). After adjustment for demographic and known risk factors for PTB, prepregnancy obesity was associated with higher odds of both early [OR 1.78 (1.19, 2.66)] and late [OR 1.49 (1.09, 2.04)] medically-induced PTB. These effect estimates were attenuated with inclusion of hypertensive disorders of pregnancy and gestational diabetes. For spontaneous deliveries, prepregnancy obesity was associated with decreased odds of PTB (0.76 [0.58, 0.98]) and underweight was nearly associated with increased odds of PTB (1.46 [0.99, 2.16]).


CONCLUSIONS
Prepregnancy obesity is associated with higher risk of medically-induced, but not spontaneous PTB. Hypertensive disorders of pregnancy and gestational diabetes appear to partially explain the association between prepregnancy obesity and early and late medically-induced PTB.
Department of Pediatrics, Boston Medical Center, Boston University School of Medicine, Boston, USA. margaret.parker@bmc.org.
Margaret G
MG
Parker
Fengxiu
F
Ouyang
Colleen
C
Pearson
Matthew W
MW
Gillman
Mandy B
MB
Belfort
Xiumei
X
Hong
Guoying
G
Wang
Linda
L
Heffner
Barry
B
Zuckerman
Xiaobin
X
Wang
D000328:Adult; D015992:Body Mass Index; D001900:Boston / Q000453:epidemiology; D002821:Chorioamnionitis / Q000453:epidemiology; D016640:Diabetes, Gestational / Q000453:epidemiology; D005260:Female; D005865:Gestational Age; D006801:Humans; D046110:Hypertension, Pregnancy-Induced / Q000453:epidemiology; D007751:Labor, Induced / Q000706:statistics & numerical data; D009765:Obesity / Q000453:epidemiology*; D011247:Pregnancy; D047928:Premature Birth / Q000453:epidemiology*; D015995:Prevalence; D012307:Risk Factors; D047929:Term Birth; D013851:Thinness / Q000453:epidemiology; D055815:Young Adult