1 California Preterm Birth Initiative, University of California, San Francisco, San Francisco, CA, United States.
2 Department of Obstetrics, Gynecology and Reproductive Sciences, Medical Center, University of California, San Francisco, San Francisco, CA, United States.
3 Department of Medicine, Yale School of Medicine, New Haven, CT, United States.
4 Department of Infectious Disease Epidemiology, London School of Hygiene and Tropical Medicine, London, United Kingdom.
5 Department of Global Health and Development, London School of Hygiene and Tropical Medicine, University of London, London, United Kingdom.