A research team finds that an experimental drug causes problems for prenatal development, but only if it is given within the first 20 days of pregnancy. Researchers are worried that this will most likely affect

a. heart development.

b. the blastocyst.

c. length of gestation.

d. the baby's birth weight

a. heart development.

Explanation:

As the third week of fetal development starts, the process of formation of blood vessels begins. Also, the splanchnic mesoderm in the head end of the embryo forms heart on days 18 and 19. Since it develops heart, this part of the mesodermal cells is called the cardiogenic area. The underlying endoderm sends the induction signals to these mesodermal cells to form a pair of endocardial tubes that gradually develop into a single primitive heart tube.

This primitive heart begins to beat by the end of the third week of development. Therefore, any drug that affects fetal development during the first 20 days of the pregnancy might affect the development of the heart.