In one of the largest studies of its kind, US researchers evaluating nearly 1600 women in Argentina, Brazil and Chile found that the children of mothers who smoke during pregnancy are at 40% higher risk of delays in neurological development during a critical phase of growth between three and 24 months of age.

http://medicalxpress.com/news/2011-08-effects-prenatal-infant-neurodevelopment-worse.html
http://www.eurekalert.org/pub_releases/2011-08/uocp-eop082211.php
http://www.medicaldaily.com/news/20110823/7056/pregnancy-sexual-health-george-wehby-neural-development-
neurology-cigaretteprenatal-smoking-mothers.htm