- Correspondence to Van T Tong, Division of Reproductive Health, National Center for Chronic Disease Prevention and Health Promotion, Centers for Disease Control and Prevention, 4770 Buford Hwy, NE, MS-F74, Atlanta, GA 30341 USA;
- Received June 14, 2013
- Accepted April 1, 2014
- First published April 30, 2014.
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