1. From the Centro de Investigaciones en Salud Poblacional, Instituto Nacional de Salud Pública, Cuernavaca, Morelos, México;
2. Universidad Autónoma de Chihuahua, México;
3. Pan American Health Organization, México;
4. Departments of Maternal and Child Health and Nutrition, Harvard School of Public Health, Boston, Massachusetts;
5. Instituto de Investigacion en Matemáticas Aplicadas y Sistemas, National Autonomous University of Mexico, Mexico;
6. American British Cowdray Hospital, Mexico; and
7. Channing Laboratory, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School and Occupational Health Program, Department of Environmental Health, Harvard School of Public Health, Boston, Massachusetts.