Objectives To determine if the anatomical severity of oral clefting affects familial recurrence in a large population based sample. To provide reliable recurrence risk estimates for oral cleft for first, second, and third degree relatives.

Design Population based cohort study.

Setting Denmark.

Participants 6776 individuals affected with an oral cleft born from 1952 to 2005 and 54 229 relatives.

Main outcome measures Recurrence risk estimates for oral cleft for first, second, and third degree relatives and stratification by severity, specificity, parent of origin effect, and family size for first degree relatives.

Results For cleft lip and palate probands we observed recurrence risks for first, second, and third degree relatives of respectively 3.5% (95% CI 3.1% to 4.0%), 0.8% (95% CI 0.6% to 1.0%), and 0.6% (95% CI 0.4% to 0.8%). Individuals affected by the most severe oral cleft had a significantly higher recurrence risk among both offspring and siblings, eg, the recurrence risk for siblings of a proband with isolated bilateral cleft lip with cleft palate was 4.6% (95% CI 3.2 to 6.1) versus 2.5% (95% CI 1.8 to 3.2) for a proband born with a unilateral defect.

Conclusions Anatomical severity does have an effect on recurrence in first degree relatives and the type of cleft is predictive of the recurrence type. Highly reliable estimates of recurrence have been provided for first cousins in addition to more accurate estimates for first and second degree relatives. These results and the majority of prior data continue to support a multifactorial threshold model of inheritance.