1. Department of Mathematics, University of Utah, 155 S 1400 E, JWB 233, Salt Lake City, UT 84112, USA.
2. Department of Biostatistics and Informatics, Colorado School of Public Health, Aurora, CO 80045, USA.
3. Department of Obstetrics and Gynecology, University of Colorado School of Medicine (AMC) Building RC2, Room P15 3103, Aurora, CO 80045, USA.