Background Maternal obesity is a well-known risk factor for both total preterm birth (PTB) and spontaneous PTB in singleton gestation, whereas this association is not well determined in multiple pregnancy. The objective of this study was to determine the risk of spontaneous PTB according to the pre-pregnancy body mass index (BMI) in twin gestations. Methods The association between the risk of PTB and pre-pregnancy BMI was determined in women pregnant with twins between 2004 and 2014. Pre-pregnancy BMI values were divided into three groups (underweightormal/overweight and obese). PTB was classified as spontaneous PTB (following preterm premature rupture of membranes, preterm labor, or cervical insufficiency) or medically indicated PTB (cesarean section or induction of labor because of maternal/fetal indications). Results A total of 1,959 women were included in the analysis, and the percentages of total PTB and spontaneous PTB were 13.1% and 9.3%. The percentages of total PTB and spontaneous PTB in three groups were 14.1%, 11.9%, 16.3%, respectively, and 11.0%, 8.0%, 12.5% ( P Conclusion The risks of both total and spontaneous PTB were significantly greater in the overweight/obese group than in the normal BMI group. Go to: Graphical Abstract
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