Teratogens affect the fetus and not the mother because teratogens are "environmental exposures" done by the mother. It is an agent that can disturb the development of an embryo of fetus such as drugs or ingestion of harmful substances such as smoking and drinking. What happens is that the toxins are being ingested by the fetus through the umbilical cord where they are being supplied by oxygen and nutrition and instead of providing nutrients harmful substances are being ingested by a growing fetus.
