Background The aetiology of congenital melanocytic naevi (CMNs) is unknown.
Objectives To identify potential aetiological factors in families of children with CMNs, and to relate these to long-term outcome measures.
Methods Three hundred and forty-nine CMN families completed questionnaires about pregnancy and parental factors, and yearly questionnaires on the health of their child and details of the CMN. Seventy-nine control families completed one set of questionnaires, excluding CMN details.
Results The mean prospective follow-up of 301 CMN families was 9·2 years, median 8·9 years, total 2679 years. Forty per cent of patients had CMNs > 20 cm projected adult size (PAS) or multiple CMNs. Twenty per cent of patients had abnormal neurodevelopment and although this was positively associated with PAS it was seen across all size categories. The rate of malignant melanoma was 1·4%. This was strongly associated with PAS with all five cases in patients with CMNs > 60 cm PAS/multiple CMNs (rate in that group 14%). Twenty-five per cent of CMN patients had a positive family history of a CMN in a second-degree relative (FHCMN). This group had a significantly different gender ratio, suggesting a different underlying mutation. Maternal FHCMN was negatively associated with PAS and satellites at birth, and maternal freckling was negatively associated with PAS. Other factors found to be significantly increased in CMN families compared with controls were maternal smoking and ill health during pregnancy. Maternal smoking was positively associated with PAS.
Conclusions This study relies on data from families after they have had a child with a CMN, and therefore may be subject to recall bias. Despite this, it contributes significantly to the knowledge of epidemiology of CMNs, and provides some important clues to the genetic basis of the condition.