Division of Epidemiology, Department of Obstetrics and Gynecology, New York University School of Medicine, 550 First Avenue, NBV 9E2, New York, NY 10016, USA. lukana01@med.nyu.edu
Epidemiology and Public Health Sciences Section, Department of Public Health and Clinical Medicine, Umea University Hospital, Building 9B, SE-901 85 Umea, Sweden. margareta.norberg@epiph.umu.se