Ascientist wants to prove that certain chemicals in cosmetics are likely to cause harm to developing fetuses, when used by a pregnant mother. what is the most likely limitation she will face in her scientific design?

The limitation is that she can't test is on humans for ethical reasons. In order to test this, theoretically, she would need to ask some women to use this cosmetic and compare their fetuses to the fetuses of women who didn't. But this is not likely to be approved by the ethical committee, as causing damage to a fetus is not ethical.