After 3 weeks of pregnancy, some drugs can enter the fetus through the placenta. This is a critical period for the development of fetal tissues and embryonic organs, and it is also a period when the fetus is highly sensitive to drugs. Improper use of medication can lead to various malformations in the fetus. Although medication in the second and third trimesters rarely causes fetal organ malformations, it may also cause organ dysfunction after birth.