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The phenotypic variability of non-syndromic cleft lip (CL) is broad. We demonstrate that the prevalence of orbicularis oris (OO) muscle anomalies, detectable only by ultrasound, is higher in first-degree relatives of individuals with overt CL than in the general population. These findings suggest that occult OO defects may be part of the spectrum of the CL phenotype, that offspring of individuals with such defects are at an increased risk to develop overt CL, and that ultrasound may be a useful tool in future population studies designed to identify CL susceptibility genes.

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