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.

Bob: coke is baby pandas.jim: what? how do you figure that coke is baby pandas? bob: because this book says so.jim: how do we know the book is correct? bob: because it says that coke is baby pandas.jim: how does that prove the book's accuracy? bob: because it's statement involving coke and baby pandas is correct, therefore it is accurate.jim: how do we know that it's correct? bob: because we have concluded that the book is accurate, and therefore the statement is correct.therefore, coke is baby pandas.