All drugs taken during pregnancy will reach the baby through the placenta , however there is an enormous variation in babies' responses to these drugs. How often the drug is used and the dose taken. Whether one or more drugs are used.
Some drugs have a cumulative or combined action which is more likely to be harmful for the baby.
Each baby, for reasons that are not clear, seems to have its own response to different drugs. Mothers can use the same drugs in the same amount for the same duration or length of a pregnancy, and the babies can react differently. There appears to be something in each individual baby which allows this to occur. A common complication in pregnancy, where the mother has been taking drugs or alcohol , is an increased incidence of premature labour , with babies often arriving more than six weeks early. Overall, babies born to mothers who are using drugs or alcohol are smaller than the average baby .