1. Department of Obstetrics and Gynecology, University of Nebraska Medical Center , Omaha, Nebraska 68198, USA
2. Veterans Affairs Nebraska-Western Iowa Health Care System , Omaha, Nebraska 68105, USA
3. Department of Genetics, Cell Biology and Anatomy, University of Nebraska Medical Center , Omaha, Nebraska 68198, USA
4. Heartland Center for Reproductive Medicine , Omaha, Nebraska 68138, USA