Final answer:
Lithium exposure in the first trimester can lead to congenital abnormalities, especially in the heart, as this is the critical development period for the fetus's internal organs.
Step-by-step explanation:
Exposure to lithium during the first trimester of pregnancy is known to increase the risk of congenital abnormalities, particularly affecting the heart. The first trimester is a critical period for the development of the fetus's internal organs and body structures. During this time, the limb buds, eyes, heart, and liver are formed, and exposure to teratogens can severely affect the emerging structures, potentially leading to congenital heart defects.