544: The relationship between body mass index, cervical length and spontaneous preterm birth

Abstract

There is conflicting evidence about the association of maternal weight with spontaneous preterm birth (SPTB), with some studies suggesting that SPTB decreases as maternal body mass index (BMI) increases and other suggesting that it increases with BMI. The objective of this study was to determine if mid-trimester cervical length (CL) mediates the relationship between BMI and SPTB. This was a retrospective cohort of women with a singleton pregnancy who underwent routine transvaginal CL assessment between 18 and 24 weeks. Women were categorized into the IOM-defined BMI categories based on BMI at the time of delivery (normal, overweight, class 1 obesity, class 2 obesity or higher). Women with prior SPTB or stillbirth in current pregnancy were excluded. Univariable comparisons of maternal characteristics (including mid-trimester CL) and BMI categories were conducted with Chi square or one way ANOVA tests followed by Scheffe multiple comparisons. The rate of SPTB was then compared across BMI categories. Multivariable logistic regression was done to determine whether BMI category was associated with SPTB independent of the CL. Of the 18,100 women included in this analysis, 2403 (13.3%) were normal weight, 7822 (43.2%) were overweight, 2962 (16.4%) were class 1 obesity, and 4913 (27.1%) were class 2 obesity or higher. In univariable analysis, increasing BMI category was associated with longer CL (4.4cm ± 0.8, 4.5cm ± 0.8, 4.5cm ± 0.8 and 4.6cm ± 0.9, p<.001). SPTB was less common among women with increasing BMI category (5.6%, 3.9%, 4.2% and 3.7%, p=.002). In multivariable regression, a higher BMI category was associated with a lower frequency of SPTB. This association persisted when CL was entered into the model as a covariate (Table). Despite all the adverse outcomes associated with high BMI and pregnancy, women with a higher BMI category had longer mid-trimester CL and, correspondingly reduced SPTB. However, the decreased risk of SPTB was not completely mediated by having a longer cervical length. The reason for the potential protective effect from prematurity is unknown and its mechanisms require further investigation.