Final answer:
Smoking, drinking alcohol, using drugs, and exposure to harmful chemicals should be avoided during pregnancy as they can harm the fetus and lead to a range of potential problems.
Step-by-step explanation:
Smoking, drinking alcohol, using drugs, and exposure to harmful chemicals are activities that can harm the fetus and should be avoided during pregnancy. These substances can cross the placenta and affect the developing fetus, leading to a range of potential problems such as cognitive deficits, developmental delays, behavioral issues, physical defects, and distinctive facial features. It is particularly important to avoid these substances during the first eight weeks after fertilization when the major organs are forming.