1. Isala Fertility Center, Isala, Dr van Heesweg 2, 8025 AB Zwolle, The Netherlands
2. WHO/HRP (the UNDP/UNFPA/UNICEF/WHO/World Bank Special Programme of Research, Development and Research Training in Human Reproduction), Avenue Appia 20, 1202 Geneva, Switzerland
3. Genk Institute for Fertility Technology, Department of Obstetrics and Gynaecology, Ziekenhuis Oost-Limburg, Schiepse Bos 6, 3600 Genk, Belgium
4. Department of Physiology, Hasselt University, Martelarenlaan 42, 3500 Hasselt, Belgium