While alcohol and tobacco use have been falling among pregnant women, the percentage who fessed up to marijuana use doubled between 2002 and 2014, to about 4 percent, federal surveys show. But the real prevalence is another unknown. Many moms keep mum because of stigma and fear of punishment. That fear is especially keen among women who have just given birth. Under federal and many state child abuse prevention laws, hospitals are supposed to notify child protective agencies about newborns who test positive for exposure to marijuana.