1. Natural Cycles USA Corp, New York, New York, USA.
2. Department of Clinical Sciences at Danderyd Hospital, Karolinska Institutet, Stockholm, Sweden.
3. Department of Epidemiology and Public Health, Khalifa University, Abu Dhabi, United Arab Emirates.
4. Department of Women's and Children's Health, Karolinska Institutet and Karolinska University Hospital, Stockholm, Sweden.