Cocaine and heroin ... causes an increase in the passage of some chemicals into the fetus.
... As complete abstinence is impossible for many people addicted to drugs who become pregnant, maintenance treatment with methadone is often used to limit damage to the developing child. However, methadone itself can also be dangerous, too much fetal exposure leading to harmful withdrawal symptoms in the newborn. ... while ... narcotics didn't increase transfer of methadone, they did allow transfer of other test substances. This suggests the barrier function of the placenta may be compromised. ... substances or bacteria and viruses may cross the placenta and harm the fetus. Previous studies have reported increased prevalence of infectious diagnoses in cocaine-exposed infants".