1. Department of Obstetrics, Fetal Medicine Unit, Vall d'Hebron Hospital, Universitat Autònoma de Barcelona, 08035-Barcelona, Spain
2. Department of Obstetrics, Son Llatze Hospital, Mallorca, Spain
3. Department of Obstetrics, Granoller Hospital, Barcelona, Spain
4. Department of Obstetrics, Terrassa Hospital, Barcelona, Spain
5. Department of Stadistics, Vall d'Hebron Hospital, Universitat Autònoma de Barcelona, 08035-Barcelona, Spain