1. Department of Obstetrics and Gynecology, Academic Medical Center, P.O. Box 22770, 1100 DE Amsterdam, The Netherlands
2. Department of Obstetrics and Gynecology, Onze Lieve Vrouwe Gasthuis, P.O. Box 95500, Amsterdam, The Netherlands
3. Clinical Research Unit, Academic Medical Center, P.O. Box 22770, Amsterdam, The Netherlands