- 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 14 June 2013
- Accepted 1 April 2014
- Published online 20 April 2015.
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