1. Instituto Nacional de Salud Pública, Septima Cerrada de Fray Pedro de Gante No 50, Col Seccion XVI, Tlalpan, Mexico City, 14000, Mexico.
2. Department of Reproductive Health and Research, World Health Organization, Geneva, Switzerland.
3. Population Council Mexico, Mexico City, Mexico.
4. Mexico City Ministry of Health, Reproductive Health Program, Mexico City, Mexico.