Abstract
Objective Studies have reported an increased risk of spontaneous preterm birth associated with
elevated prepregnancy body mass index (BMI) among nulliparous but not multiparous
women. We examined whether changes in BMI and weight between pregnancies contributed
to risk of preterm birth among obese (BMI > 29 kg/m2) women.
Study Design This study utilized maternally linked California birth records of sequential singleton
births between 2007 and 2010. Preterm birth was defined as 20 to 31 or 32 to 36 weeks
of gestation. BMI was examined as category change and by tertile of weight change.
Primary analyses included women without diabetes or hypertensive disorders; these
women were compared with those without prior preterm birth, women with preterm deliveries
preceded by spontaneous preterm labor, and women without any exclusions (i.e., diabetes
or hypertensive disorders).
Results Analyses showed that obesity was not associated with increased risk of spontaneous
preterm birth among multiparous women. Women whose BMI increased had a decreased risk
of spontaneous preterm birth at 32 to 36 weeks. Change in BMI or weight between pregnancies
did not substantively alter results.
Conclusion Among multiparous women, obesity was associated with reduced risk of spontaneous
preterm delivery. This observed association is complex and may be influenced by maternal
age, gestational age, placental insufficiency, and altered immune response.
Keywords
body mass index - preterm birth - obesity - pregnancy
