After collection of blood, serum was stored at -70ºC until the measurement of AMH level. Serum levels of AMH were determined by an enzyme-linked immunosorbent assay using commercial kit (Beckman Coulter Inc., Paris, France). The detection limit of the assay was 0.14 ng/mL, and the intra- and inter-assay coefficients of variation were 12.3% and 14.2%, respectively.