Huntington's disease (HD) is an adult onset, autosomal dominant disorder1 with onset of symptoms usually in the fourth or fifth decade. The classical triad of clinical features, movement disorder, cognitive impairment, and personality and psychiatric disorder, cause serious management problems. There is significant morbidity within the affected families, especially for those who themselves are at risk of developing the disease. HD affects about 5000 people in the UK and about five times that number are considered to be at 50% risk of developing the disease.

Since the mapping of the locus for Huntington's disease on chromosome 4 in 1983,2 followed by the identification of the gene and its expanded polyglutamine repeat in HD in 1993,3 it has been possible to offer accurate tests for HD. Prenatal tests and presymptomatic predictive tests for adults at risk for HD are available at genetic centres throughout the world.

There are two common approaches to prenatal testing in HD. Direct testing involves investigating for the presence of the mutation in a pregnancy. This gives an accurate result. If the status of the at risk parent has not been ascertained, then this may produce predictive information about that person.

In exclusion testing, the at risk grandparental chromosome 4 locus is excluded using linkage analysis. This test preserves the 50% risk of the parent, and allows a pregnancy at low risk to continue. In this situation, pregnancies that share the risk of the parent would be terminated. However, should the at risk parent not develop HD, a normal pregnancy would have been lost.

Given the technical feasibility of prenatal mutation testing and the severity of the disorder, it might be expected that prenatal diagnosis would be frequently requested.

Tyler et al 4 reviewed a group of referrals for exclusion testing in pregnancy, and …