The systemic endocrine environment during the later stages of follicle development has a crucial role in co-ordinating follicular and oocyte maturation before ovulation. Polycystic ovary syndrome (PCOS) is associated with abnormal circulating hormones, abnormal peri-follicular vascularity and significant abnormalities of granulosa cell function. After induction of ovulation, fertilization rates in vivo in women with PCOS are normal, but there is an increased risk of early pregnancy loss, particularly in obese patients. After in-vitro maturation of oocytes or following ovulation induction for IVF, oocyte and embryo quality in vitro are not obviously impaired in PCOS. In some reports however, specific endocrine abnormalities, such as hyperinsulinaemia/insulin resistance, have been noted to be associated with reduced fertilization rates and abnormal early embryonic development.