Part 1. Understanding the embryology and genetics of cleft lip and palate.

Craniofacial birth defects are the fourth most common congenital anomaly in newborns. Cleft lip and cleft palate (CL/CP) are the most common and immediately recognizable craniofacial anomalies. Some are diagnosed prenatally on ultrasound; more often, they are first noted in the delivery room. The infant may have a cleft that is unilateral, bilateral, complete, or incomplete, and it may involve the lip only, the palate only, or both. Cleft lip and/or cleft palate are often isolated nonsyndromic occurrences; however, when associated with other abnormal physical findings, a recognizable syndrome may be present. Part 1 of this 2-part article describes the incidence of CL/CP. The embryology of the face, lip, and palate are reviewed to enhance the understanding of the timing, complexity, and factors that may influence the development of this lesion. The emerging genetic links, environmental influences, and potential teratogens that may interact to contribute to CL/CP are discussed. Part 2 of this series will provide clinicians with tools to perform a focused risk assessment and obtain a detailed family and pregnancy history to evaluate for known associated risk factors for CL/CP. The article provides a guide for a systematic physical examination of infants with CL/CP. Careful assessment for other midline defects or physical findings consistent with associated syndromes is also discussed. Pictorial examples of a variety of forms of CL/CP are provided to enhance understanding of the spectrum of this defect. Treatment and long-term complications of CL/CP are reviewed with an emphasis on family support, identifying educational resources, and counseling.