http://www.sciencedirect.com/science/article/pii/S0002937806010064

Effects of birth spacing on maternal health: a systematic review
Agustin Conde-Agudelo MD, MPH1, Anyeli Rosas-Bermúdez MPH2 and Ana C. Kafury-Goeta MD3

American Journal of Obstetrics and Gynecology

Volume 196, Issue 4, April 2007, Pages 297-308