Final answer:
Environmental factors have the greatest effect on an embryo during the early stages of pregnancy because this is when all the major organs are forming and exposure to harmful substances during this time could damage the developing body systems.
Step-by-step explanation:
Environmental factors have the greatest effect on an embryo during the early stages of pregnancy because this is when all the major organs are forming.
Exposure to harmful substances during this time could damage the developing body systems.
Additionally, a fetus whose mother faces famine or infections during pregnancy will receive different environmental signals compared to a fetus that receives an appropriate food supply and is not exposed to infections.