1. Department of Obstetrics and Gynecology, Royal Victoria Hospital, Research Institute of McGill University Health Centre, Montreal, CanadaH4A 3J1
2. Department of Epidemiology, Biostatistics, and Occupational Health, McGill University, Montreal, Canada H3A 1A2
3. Biostatistics and Computational Biology Branch, National Institute of Environmental Health Sciences, Research Triangle Park 27709, USA
4. Social & Scientific Systems Inc., Durham, NC 27703, USA
5. Epidemiology Branch, National Institute of Environmental Health Sciences, Research Triangle Park 27709, USA