1. Department of Gastroenterology and Hepatology, VU University Medical Centre (VUmc), PO Box 7075, 1081 HZ, Amsterdam, The Netherlands
2. Division of Reproductive Medicine, Department of Obstetrics and Gynaecology, VU University medical centre (VUmc), PO Box 7075, 1007 MB, Amsterdam, the Netherlands
3. Department of Epidemiology and Biostatistics, VU University (VU), Medical Faculty (F-wing), 1081 HV, Amsterdam, The Netherlands