Some teratogens have a threshold effect, which means that they are

Teratogens are catalysts that can cause birth defects in fetuses. The threshold effect means that this agent is virtually harmless until exposure to it reaches a toxic level, or in some cases would affect the fetus if it was at a lower birth weight, or if a pregnant mother fails to gain sufficient weight during the pregnancy or may be simply be harmful to some organs if exposure occurred at a time when those organs were developing.