Objective Recurrent pregnancy loss(RPL)presents a formidable challenge for individuals undergoing in vitro fertilization-embryo transfer(IVF-ET),forming both a clinical dilemma and a focal point for scientific inquiry.This study endeavors to investigate the intricate interplay between clinical features,such as age,body mass index(BMI),and waist-to-hip ratio(WHR),and routine laboratory parameters,including sex hormones,blood composition,liver and thyroid functions,thyroid antibodies,and coagulation indicators,in RPL patients undergoing IVF-ET.By meticulously analyzing these variables,we aim to uncover the latent risk factors predisposing individuals to RPL.Identifying potential factors such as advanced maternal age,obesity,and insulin resistance will provide clinicians with vital insights and empirical evidence to strengthen preventive strategies aimed at reducing miscarriage recurrence.Methods This retrospective case-controlled study included RPL patients who underwent IVF-ET treatment at Sun Yat-sen Memorial Hospital,Sun Yat-sen University,between January 2012 and March 2021 as the case cohort,compared with women receiving assisted reproductive treatment due to male infertility as the control cohort.The fasting peripheral blood was collected 5 days before the first menstrual cycle at least 12 weeks after the last abortion.The clinical characteristics and relevant laboratory indexes of the two groups were compared.Employing both univariate and multivariate logistic regression analyses,we sought to unearth potential high-risk factors underlying RPL.Additionally,a linear trend analysis was conducted to assess the linear relationship between total testosterone(TT)levels and the number of miscarriages.Results In contrast to the control cohort,the RPL cohort exhibited significant increases in age,BMI,and WHR(P<0.05).Notably,TT levels were markedly lower in the RPL cohort(P=0.022),while no significant differences were observed between the two groups concerning basal follicle-stimulating hormone,luteinizing hormone,estradiol,progesterone,prolactin levels,and anti-Müllerian hormone levels(P>0.05).Moreover,fasting insulin(FINS)levels and HOMA-IR index were notably elevated in the RPL cohort relative to the control cohort(P<0.001),although no significant differences were observed in fasting blood glucose levels(P>0.05).Furthermore,the neutrophil(NEU)count and NEU-to-lymphocyte ratio were notably higher in the RPL cohort(P<0.01).Univariate logistic regression analysis identified several factors,including age≥35 years old,BMI≥25 kg/m2,WHR>0.8,FINS>10 mU/L,HOMA-IR>2.14,NEU count>6.3×109 L-1,and an elevated NEU/lymphocyte ratio(NLR),as significantly increasing the risk of RPL(P<0.05).Although TT levels were within the normal range for both cohorts,higher TT levels were associated with a diminished RPL risk(odds ratio[OR]=0.67,95% confidence interval[CI]:0.510-0.890,P=0.005).After adjustments for confounding factors,age≥35 years old(OR=1.91,95% CI:1.06-3.43),WHR>0.8(OR=2.30,95% CI:1.26-4.19),and FINS>10 mU/L(OR=4.50,95% CI:1.30-15.56)emerged as potent risk factors for RPL(P<0.05).Conversely,higher TT levels were associated with a reduced RPL risk(OR=0.59,95% CI:0.38-0.93,P=0.023).Furthermore,the linear trend analysis unveiled a discernible linear association between TT levels and the number of miscarriages(Ptrend=0.003),indicating a declining trend in TT levels with escalating miscarriage occurrences.Conclusion In patients undergoing IVF-ET,advanced maternal age,lower TT levels,increased WHR,and elevated FINS levels emerged as potent risk factors for RPL.These findings provide clinicians with valuable insights and facilitate the identification of patients who are at high risks and the formulation of preventive strategies to reduce the recurrence of miscarriages.