1. Department of Obstetrics & Gynaecology, University of Groningen, Groningen, the Netherlands
2. Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada
3. PET Center, University of Groningen, Groningen, the NetherlandsCorrespondence: Arie Nieuwenhuizen, Department of Obstetrics & Gynaecology, University of Groningen, Hanzeplein 1, 9713 GZ Groningen, the Netherlands.