1. Research Unit in Reproductive Medicine, Teaching and Research Program, and Maternal-Fetal Medicine Department, UMAE en Ginecologia y Obstetricia “Luis Castelazo Ayala,” Instituto Mexicano del Seguro Social, Mexico D F, Mexico
2. Unidad de Investigacion Medica en Medicina Reproductiva, UMAE en Ginecologia y Obstetricia, IMSS, Apartado Postal 99-065, Unidad Independencia, 10101 México D. F.