Can hard drug abuse cause mental retardation in a child even if there wasn't any drug use during the actual pregnancy?

Early pregnancusage. If drugs are used before noticing pregnancy and then stopping usage would create mental retardation. Also nutritional deficiency due to drug usage affects childs brain development and indirectly contributes to mental illness.
Usually not. Most drugs require transfer to the baby across the placenta. Some previous drug use can affect the sperm; however, they tend to affect fertility and sometimes growth of the fetus rather than cause mental retardation.