During the pre-implantation period, the embryo is less susceptible to teratogens because it consists of a relatively small number of cells that are totipotent. This means that each cell has the potential to develop into any type of cell in the body. If one or more cells are damaged by a teratogen, the other cells can compensate and continue to develop normally.

After implantation, the embryo becomes more susceptible to teratogens. This is because the cells begin to differentiate and specialize into different types of cells. If a particular cell is damaged by a teratogen, it may not be able to carry out its specialized function, which can lead to a birth defect.

The type of birth defect that occurs depends on the specific teratogen and the stage of development at which the embryo is exposed. Some teratogens can cause multiple birth defects, while others may only cause a single defect.

It is important for women to avoid exposure to known teratogens during pregnancy, especially during the first trimester when the embryo is most vulnerable.