Heavy, long-term exposure to marijuana may have biologically based physical, mental, behavioral and social health consequences and may be "associated with diseases of the liver (particularly with co-existing hepatitis C), lungs, heart, and vasculature". Mothers who used marijuana during pregnancy have children with elevated levels of depression, hyperactivity, impulsivity and inattention. It is recommended that cannabis use be stopped before and during pregnancy as it can result in negative outcomes for both the mother and baby. However, maternal use of marijuana during pregnancy does not appear to be associated with low birth weight or early delivery after controlling for tobacco use and other confounding factors. A 2014 review found that while cannabis use may be less harmful than alcohol use, the recommendation to substitute it for problematic drinking was premature without further study. Various surveys conducted between 2015 and 2019 found that many users of cannabis substitute it for prescription drugs (including opioids), alcohol, and tobacco; most of those who used it in place of alcohol or tobacco either reduced or stopped their intake of the latter substances.