Which embryologic fusion failure leads to a cleft lip?

• A. Failure of fusion between palatal shelves with each other during weeks 7–12.
• B. Failure of fusion between the maxillary process and the medial nasal process during early facial development.
• C. Abnormal development of the mandible.
• D. Excessive growth of the maxillary prominence.

Answer: (B)

Cleft lip happens when the facial build-up doesn’t come together in the right way early in development. Specifically, the maxillary process fails to fuse with the medial nasal process during the first weeks of gestation. This fusion normally creates the upper lip, so when it doesn’t occur, a gap appears in the lip that can be unilateral or, less commonly, bilateral and may extend into the nose.

Understanding the timing helps distinguish this from other cleft types. The palate forms later, as the palatal shelves from the maxillary processes elevate and fuse with each other and with the nasal septum around weeks 7–12. If those shelves don’t fuse properly, the result is a cleft palate, not a cleft lip.

Abnormal mandible development isn’t the typical cause of a cleft lip, though it can be associated with other facial anomalies. Excessive growth of the maxillary prominence wouldn’t produce a cleft; it would alter contour rather than leave a gap from failed fusion.