Which fetal defect is lithium associated with during pregnancy?

Lithium is primarily associated with an increased risk of Ebstein's anomaly when taken during pregnancy. Ebstein's anomaly is a rare congenital heart defect that affects the tricuspid valve and the right side of the heart. The use of lithium, a medication commonly prescribed for bipolar disorder, during the first trimester of pregnancy has been linked to this specific cardiac defect. The mechanism by which lithium causes this anomaly is thought to be related to its effects on fetal development, particularly regarding the heart and its structural formation.

The other options do not have a significant association with lithium use during pregnancy. Microcephaly is often linked to other teratogens, like Zika virus infection. Spina bifida is primarily associated with folic acid deficiency and specific genetic factors rather than lithium. Cleft palate is associated with a range of teratogens, but lithium is not one of the primary medications linked to this defect. Therefore, Ebstein's anomaly is the correct association regarding lithium exposure in pregnancy.