Many epidemiologic studies showed a plausible link between low birth weight and chronic metabolic disorders manifested as hypertension, diabetes and obesity later in life, suggestive of an early fetal programming. There is evidence to support fetal origin of PCOS. Female rhesus monkeys that were exposed to excess androgen in utero, were born smaller for gestational age. These animals had many of the reproductive features of PCOS, including increased LH levels, irregular ovulation, polycystic ovaries and functional ovarian hyperandrogenism. Similarly, in retrospective cohort studies, girls with elevated adrenal androgen levels or with PCOS were significantly smaller for gestational age at birth than reproductively normal control girls, suggestive of a possible fetal origin for some features of PCOS in human studies. Molecular mechanism for fetal programming is not clearly understood, but permanent changes in gene expression caused early insult may be a factor.