With respect to toxic substances and pregnancy,what is meant by a critical period of development?

Question

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A critical period of development is a specific time during embryonic or fetal development when an organism is most susceptible to harm from exposure to a toxic substance. During this time, even small doses of a toxic substance can have a significant impact on the growth and development of the fetus. It is important to know when these critical periods occur so that pregnant mothers can take steps to avoid exposure to toxic substances during these times.

With Respect to Toxic Substances and Pregnancy,what Is Meant by a Critical