Key Laboratory of Reproductive Medicine of Guangdong Province, Key Laboratory for Major Obstetric Diseases of Guangdong Province, Key Laboratory of Reproduction and Genetics of Guangdong Higher Education Institutes, The Third Affiliated Hospital of Guangzhou Medical University, Duobao Road 63, Guangzhou, 510150, Guangdong, People’s Republic of China. Tel: +86-20-81292292. Fax: +86-20-81292013