The polycystic ovary syndrome (PCOS) is associated with an increased rate of early pregnancy loss (EPL). Hyperinsulinemia is an independent risk factor for EPL and has been found to decrease levels of glycodelin and IGF binding protein-1 (IGFBP-1), two major endometrial proteins. We hypothesized that serum glycodelin IGFBP-1 concentrations would be reduced in women with PCOS during the first trimester of pregnancy. Fasting serum insulin, glycodelin, and IGFBP-1 were measured, and oral glucose tolerance tests were performed in 72 women with PCOS and 62 normal women. Each woman was seen once and assigned to one of three gestational groups: wk 3-5, 6-8, and 9-11. The insulin sensitivity index during oral glucose tolerance test was lower in women with PCOS compared with normal women throughout the first trimester (P < 0.0001). Both serum glycodelin and IGFBP-1 were markedly lower in women with PCOS (for glycodelin: wk 3-5, P < 0.0001; wk 6-8, P = 0.03; wk 9-11, P = 0.19; and for IGFBP-1: wk 3-5 and 6-8, P < 0.0001; wk 9-11, P = 0.0003). Comparing women with PCOS who experienced EPL with those who did not, serum glycodelin was significantly lower during wk 3-8 (P < 0.02) and serum IGFBP-1 during wk 9-11 (P = 0.003). During the first trimester, serum glycodelin and IGFBP-1 concentrations are markedly decreased in PCOS, implicating endometrial epithelial and stromal dysfunction during periimplantation and early pregnancy as a possible mechanism for EPL in PCOS. These decreases are likely to be secondary to hyperinsulinemia and reduced insulin sensitivity.