Association between body roundness index and pregnancy outcomes of in vitro fertilization

Abstract STUDY QUESTION Is there an association between the body roundness index (BRI) and pregnancy outcomes of IVF, and can it provide complementary information beyond body mass index (BMI)? SUMMARY ANSWER Higher BRI was associated with a lower likelihood of live birth following IVF, particularly among women with normal BMI. WHAT IS KNOWN ALREADY BMI is widely used to assess obesity-related reproductive risk but does not adequately characterize body fat distribution. BRI, a novel anthropometric index that may better reflect central adiposity, has been associated with cardiometabolic risk, but its relationship with IVF outcomes remains unclear. STUDY DESIGN, SIZE, DURATION This study was a secondary analysis of data from two multicentre randomized controlled trials (RCTs) in China that evaluated live birth rates between freeze-only and fresh embryo transfers. The first trial enrolled 1508 women with polycystic ovary syndrome from 14 centres between June 2013 and May 2014, with follow-up completed in July 2015. The second trial recruited 2157 women with regular menstrual cycles from 20 centres between March 2015 and November 2015, with follow-up completed in March 2017. PARTICIPANTS/MATERIALS, SETTING, METHODS After exclusions, 3523 women who underwent embryo transfer and had complete BRI data were included in the analysis. BRI was calculated from height and waist circumference measured prior to IVF. In the absence of universally accepted clinical thresholds for BRI, the participants were categorized into three groups according to the 33rd and 66th percentiles: <2.54 (n = 1171), 2.54 to < 3.54 (n = 1174), ≥3.54 (n = 1178). The primary outcome was live birth, which was defined as the delivery of neonate(s) with signs of life at ≥ 28 weeks’ gestation. Modified Poisson regression with robust variance was used to estimate adjusted risk ratios (aRRs) and 95% confidence intervals (CIs). The models were adjusted for age (continuous), duration of infertility (continuous), cause of infertility, type of embryo transfer, diagnosis of polycystic ovary syndrome (PCOS), number of oocytes retrieved (continuous), and number of embryo(s) transferred. These covariates were selected based on clinical relevance and prior literature to reduce potential confounding. MAIN RESULTS AND THE ROLE OF CHANCE The rate of live birth was lower among women in the highest BRI group (group 3) compared with those in the lowest group (group 1) (45.2% vs 53.6%; aRR, 0.89; 95% confidence interval [CI], 0.81–0.97; P=0.008). Women in group 3 also had an increased risk of total pregnancy loss compared with those in group 1 (aRR, 1.31; 95% CI, 1.07–1.61; P=0.009). In BMI-stratified analyses, higher BRI was associated with a reduced likelihood of live birth (group 2, aRR, 0.89; 95% CI, 0.81–0.98; P=0.019; group 3, aRR, 0.84; 95% CI, 0.73–0.96; P=0.009) among women with normal BMI (18.5 to < 24 kg/m2). Furthermore, each one-unit increase in BRI was associated with a 7% lower likelihood of live birth (aRR, 0.93; 95% CI, 0.87–0.98; P = 0.009) among women with normal BMI. No significant association between BRI and live birth was observed among women who were overweight or obese (BMI ≥24 kg/m2). LIMITATIONS, REASONS FOR CAUTION In this post-hoc analysis, causality cannot be established, and residual confounding cannot be ruled out. The small-sized subgroups, particularly among women with overweight or obesity, may have limited statistical power. WIDER IMPLICATIONS OF THE FINDINGS These findings suggest that BRI may capture adiposity-related risk not reflected by BMI alone, particularly among women with normal BMI. BRI may serve as a complementary anthropometric measure alongside BMI for identifying women at higher risk of adverse IVF outcomes. Further studies are needed to validate these findings across BMI categories and clarify the clinical utility of BRI in IVF populations. FUNDING This study was supported by the National Key Research and Development Program of China (2023YFC2705502) and the National Natural Science Foundation of China (82495194 and 82421004). DISCLOSURES There are no conflicts of interest to declare. TRIAL REGISTRATION NUMBER N/A.