A reasonably respectable German study shows that:
"There is strong evidence that THC, the main psychoactive component of marijuana, crosses the placenta, that maternal marijuana abuse results in intrauterine growth retardation and that infants exposed to marijuana exhibit a temporary syndrome that includes lethargy and decreased muscle tone. Fetal exposure to THC can also result in attention deficits, learning disabilities and behavioral problems. "
Which probably will come as no surprise - either on a common sense level, or considering that another study
found that endocannabinoids are significant in brain development.