Oral clefts, including cleft lip with or without cleft palate and cleft palate only, have a high rate of familial recurrence compared with many birth defects.1 The cleft can occur in association with other congenital abnormalities, sometimes as part of an underlying recognisable syndrome, or more often as an isolated defect. Inheritance is complex and related to environmental and genetic factors.2 Although a genetic component exists, the precise genetic basis is unclear.

The risks of having a child with an oral cleft that are used when counselling families at increased risk are based on empirical figures derived from studies that have several limitations, such as inclusion of syndromic cases, the grouping of all oral clefts together, incomplete ascertainment, and a lack of longitudinal data.

In their accompanying paper, Sivertson and colleagues report a longitudinal population based study of the risk of non-syndromic oral clefts in Norwegian families.3 They analyse the type and the severity of the cleft in the index case. Norway has an excellent model for generating useful epidemiological data for oral …