1. Division of Maternal-Fetal Medicine and Perinatal Research, Department of Obstetrics and Gynecology, The University of Texas Medical Branch at Galveston, Galveston, TX 77555, USA
2. Department of Biochemistry and Molecular Biology, College of Medicine, University of the Philippines Manila, Manila, 1000, Philippines
3. Department of Epidemiology and Biostatistics, College of Public Health, Temple University, Philadelphia, Pennsylvania