The importance of the 'uterine factor' in recurrent pregnancy loss: a retrospective cohort study on women screened through 3D transvaginal ultrasound

Study question: What is the prevalence of congenital and acquired anomalies of the uterus in women with recurrent pregnancy loss (RPL) of unknown etiology examined using 3D transvaginal ultrasound (US)? Summary answer: Depending on the adopted diagnostic criteria, the prevalence of partial septate uterus varies between 7% and 14% and a T-shaped uterus is 3% or 4%, while adenomyosis is 23%, at least one of type 0, type 1 or type 2 myoma is 4%, and at least one endometrial polyp is 4%. What is known already: ESHRE and the Royal College of Obstetricians and Gynaecologists guidelines on RPL recommend the adoption of the 3D transvaginal US to evaluate the 'uterine factor'. Nevertheless, there are no published studies reporting the prevalence of both congenital and acquired uterine anomalies as assessed by 3D transvaginal US and diagnosed according to the criteria proposed by the most authoritative panels of experts in a cohort of women with RPL. Study design, size, duration: This was a retrospective cohort study including 442 women with at least two previous first-trimester spontaneous pregnancy losses (i.e. non-viable intrauterine pregnancies), who referred to the obstetrics and gynecology unit of two university hospitals between July 2020 and July 2023. Participants/materials, setting, methods: Records of eligible women were reviewed. Women could be included in the study if: they were between 25 and 42 years old; they had no relevant comorbidities; they were not affected by infertility, and they had never undergone ART; they and their partner tested negative to a comprehensive RPL diagnostic work-up; and they had never undergone metroplasty, myomectomy, minimally invasive treatments for uterine fibroids or adenomyomectomy. Expert sonographers independently re-analyzed the stored 2- and 3D transvaginal US images of all included patients. Congenital uterine anomalies (CUAs) were reported according to the American Society for Reproductive Medicine (ASRM) 2021, the ESHRE/European Society for Gynaecological Endoscopy (ESGE) and the Congenital Uterine Malformation by Experts (CUME) criteria. Acquired uterine anomalies were reported according to the International Federation of Gynecology and Obstetrics (FIGO) and the Morphological Uterus Sonographic Assessment (MUSA) criteria. Main results and the role of chance: The partial septate uterus was diagnosed in 60 (14%; 95% CI: 11-17%), 29 (7%; 95% CI: 5-9%), and 47 (11%; 95% CI: 8-14%) subjects, according to the ESHRE/ESGE, the ASRM 2021, and the CUME criteria, respectively. The T-shaped uterus was diagnosed in 19 women (4%; 95% CI: 3-7%) according to the ESHRE/ESGE criteria and in 13 women (3%; 95% CI: 2-5%) according to the CUME criteria. The borderline T-shaped uterus (diagnosed when two out of three CUME criteria for T-shaped uterus were met) was observed in 16 women (4%; 95% CI: 2-6%). At least one of FIGO type 0, type 1, or type 2 myoma was detected in 4% of included subjects (95% CI: 3-6%). Adenomyosis was detected in 100 women (23%; 95% CI: 19-27%) and was significantly more prevalent in women with primary RPL and in those with three or more pregnancy losses. At least one endometrial polyp was detected in 4% of enrolled women (95% CI: 3-7%). Limitations, reasons for caution: The absence of a control group prevented us from investigating the presence of an association between both congenital and acquired uterine anomalies and RPL. Second, the presence as well as the absence of both congenital and acquired uterine anomalies detected by 3D US was not confirmed by hysteroscopy. Finally, the results of the present study inevitably suffer from the intrinsic limitations of the adopted classification systems. Wider implications of the findings: The prevalence of CUAs in women with RPL varies depending on the classification system used. For reasons of clarity, the US reports should always state the name of the uterine anomaly as well as the adopted classification and diagnostic criteria. Adenomyosis seems to be associated with more severe forms of RPL. The prevalence rates estimated by our study as well as the replicability of the adopted diagnostic criteria provide a basis for the design and sample size calculation of prospective studies. Study funding/competing interest(s): No specific funding was used. The authors have no conflicts of interest to declare.