Polycystic ovary syndrome (PCOS) is a common endocrine disorder characterized by insulin resistance and hyperandrogenism. The interaction of these factors might result in increased risks of miscarriage and pregnancy complications such as gestational diabetes mellitus (GDM). To examine the pregnancy risks in women with PCOS, we compared obstetrical outcomes between patients with and without PCOS. We also studied the differences in maternal characteristics, glucose intolerance and pregnancy complications between PCOS patients with and without GDM, with and without obesity, and between successful pregnancies and miscarriages. We observed a high incidence of GDM and prevalence of GDM diagnosis in the first trimester in PCOS. Patients with GDM had higher body mass index (BMI) and lower homeostasis model assessment of beta-cell function (HOMA-beta) at preconception than those without GDM. Obese pregnant women with PCOS demonstrated a high incidence of GDM with severe insulin resistance, including high fasting insulin, HOMA of insulin resistance (HOMA-IR), and HOMA-beta at preconception compared with normal-weight patients. BMI was significantly correlated with HOMA-IR or HOMA-beta, and both indices were lower in PCOS patients with than without GDM for the same BMI. There were no significant differences in maternal characteristics (excluding maternal age) between PCOS patients with successful pregnancy and PCOS patients with miscarriages. Our data suggest that pregnant women with PCOS have an increased risk of GDM, especially if they have obesity and/or poorer insulin secretion. Measure of beta-cell function, such as HOMA-beta, at preconception might be a useful predictor of the risk of GDM in pregnant PCOS patients.