Objectives: A return to age-expected fecundity in PCOS patients is now possible using many therapeutic modalities. However, a high early miscarriage (SAB) rate continues to occur in PCOS patients following ovulation induction. This study investigated whether there are measurable endometrial/decidual characteristics of early clinical pregnancy which differ between PCOS and other infertility patients which may explain this higher SAB rate.Design: All patients achieving pregnancy at this referral center undergo trans-vaginal ultrasonography between 6 weeks, 3 days and 7 weeks, 3 days gestation by last menstrual period, adjusted for ovulation. Measurements of gestational sac, crown rump length, and depth of decidual reaction, as described below, are obtained. This is a descriptive study.Materials and Methods: All patients achieving pregnancy at this referral center undergo TVS using an Acuson 128 with 7.5 MHz transducer between 6 weeks, 3 days and 7 weeks 3 days gestation by adjusted last menstrual period. Decidual reaction is measured by first obtaining the largest 2 dimensional gestational sac size on the screen with the fetus in view and the probable implantation site identified. At the farthest point from the implantation site, the highly echogenic decidual rim of tissue is identified and measured. Prior to statistical analysis, the following groups of pregnant patients were eliminated from both patient groups: recurrent pregnancy loss, multiple gestation, IVF/GIFT/ZIFT/donor oocyte pregnancies, patients who conceived following ovarian cautery, and patients with inevitable abortion (anembryonic, no cardiac activity, etc.). PCOS patients included only clomiphene resistant anovulatory patients. Comparison patients included those undergoing controlled ovarian hyperstimulation with intrauterine insemination for endometriosis, unexplained or age-related infertility. Unpaired t-test was performed after data were ascertained to be normally distributed.Results: Over 18 months, 72 consecutive patients (21 PCOS and 49 non-PCOS) met the above criteria and underwent mensuration using TVS. Mean decidual reaction (DR) in PCOS patients was 4.10 mm (S.D. = 0.23) and in non-PCOS patients was 5.33 mm (S.D. = 0.19) (p<0.0001). Only 10% of PCO patients achieved a DR of 5.5 or greater. Conversely, only 11% of non-PCOS patients had a DR less than 4.1 mm.Conclusions: These results demonstrate a reliably present difference in the uterus during early clinical gestation in PCOS infertility patients. Depth of sonographically determined decidualization may provide an anatomic correlate, if not a basis, for the higher miscarriage rate observed in PCOS patients once pregnancy is achieved. It remains to be determined how differences in the depth of the decidual reaction reflect altered follicular or systemic endocrine environments, which in turn influence embryo quality, endometrial receptivity or implantation characteristics.