Most teratogens are most harmful to the developing fetus during the first trimester because
A) the fetus has not gathered sufficient resources to combat the effects of teratogens.
B) the placental barrier has not yet fully developed.
C) the fetal immune system is not yet capable of mounting a response to the teratogenic substance.
D) the first trimester contains the critical period of development for many major systems and body structures.

Question

Quizplus · Accepted Answer

During the first trimester, the critical period of development for many major systems and body structures, such as the brain and heart, occurs. Any exposure to teratogens during this time can have lasting and potentially devastating effects on the fetal development. The placental barrier starts to develop early in pregnancy and becomes more effective as pregnancy progresses. The immune system of the fetus also develops throughout pregnancy, but even a fully developed immune system may not be able to combat the effects of some teratogens. The resources available to the fetus may also be a factor, but the critical period of development is the primary reason why teratogens are most harmful during the first trimester.

Most Teratogens Are Most Harmful to the Developing Fetus During