Fetal diagnosis of cystic fibrosis

Two different methods of prenatal diagnosis of CF are currently available. Both are usually only suitable for couples with a 1 in 4 risk of bearing an affected child. The method of choice is first trimester diagnosis on a chorionic villus sample using linked DNA probes. The advantages are: 1. The very high degree of accuracy of diagnosis, 2. The opportunity of first trimester termination of pregnancy when the fetus is affected. The disadvantages are: 1. The need for a blood sample from an index affected child to establish phase, 2. The requirement for early workup of the nuclear family to ascertain informativeness, 3. The chance that some families will be uninformative or only partially informative for available DNA probes. An alternative form of diagnosis is that using microvillar enzyme testing on second trimester amniocentesis samples. The advantages are: 1. It is suitable for all pregnancies with a 1 in 4 risk of CF, 2. No prior workup of the family is required, 3. Assays can be completed within one hour of receipt of the sample. The disadvantages are: 1. The accuracy of diagnosis leaves some scope for error, 2. If termination of pregnancy is indicated this can only be carried out at about 18 or 19 weeks of gestation.