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The aim of this study was to investigate the relationships between uterine size and volume and clinical pregnancy rate.This longitudinal study was conducted among patients undergoing assisted reproduction technology (ART) treatment at the Reproductive Medicine Center from January 2010 to May 2017, all of whom provided informed consent to participate in the study. The uterine size, for all patients, was measured by transvaginal ultrasonography before ovarian stimulation. Clinical pregnancy was diagnosed by ultrasound confirmation of at least an intrauterine gestational sac and fetal cardiac activity 4 weeks after embryo transfer.A total of 11,924 patients were enrolled in this study. Compared to patients with uterine lengths of 50 to 59 mm (referent), patients with uterine lengths ≥60 mm had a lower clinical pregnancy rate. Compared to patients with uterine widths of 30 to 39 mm (referent), patients with uterine widths of 40 to 49 mm and those with uterine widths of ≥50 mm had a lower clinical pregnancy rate. Compared with those with a uterine anteroposterior diameter of <30 mm (referent), patients with uterine anteroposterior diameters of ≥50 mm had a lower clinical pregnancy rate. Compared with those with a uterine volume of 30 to 49 mL (referent), patients with a uterine volume ≥70 mL had a lower clinical pregnancy rate.The patients with an optimal uterine length, width, anteroposterior diameter, and volume had a higher clinical pregnancy rate than those with suboptimal uterine measurements. Uterine sizes and volumes that were too large reduced the clinical pregnancy rate.