1. Department of Obstetrics and Gynecology, University Medical Center, Groningen, University of Groningen, Groningen, The Netherlands
2. Department of Obstetrics and Gynecology, Nij Smellinghe Hospital, Drachten, The Netherlands
3. Department of Neuroscience, Section Anatomy, University Medical Centre Groningen, University of Groningen, Groningen, The Netherlands