Second trimester prenatal diagnosis of the fragile X.

The fra(X) chromosome was detected in 5 samples of amniotic fluid cells in a series of 23 pregnancies at risk. The prenatal results were confirmed in 2 male abortuses, one with a relatively high and one with a very low frequency of expression in both amniocytes and fetal tissue. In a third male fetus with low expression in amniocytes, the fra(X) was not detected in the fetal tissues tested. In another male with low expression in amniocytes the fra(X) was not detected after birth. In one female with a low expression in amniocytes, a very high frequency (28%) was detected in cord blood after birth. Low expression of the fra(X) was found in a 4-year-old normally developed girl, where the prenatal results had been negative. In 4 males and 4 females the negative prenatal diagnoses were confirmed after birth. This study indicates that prenatal diagnosis of the fragile X after amniocentesis may be complicated, either due to technical problems related to the use of amniotic fluid cells, or due to genetic heterogeneity, or both. Part of this heterogeneity could be due to the existence of normal male transmitters. Also, it seems that the frequency of expression in amniocytes from female carriers can not be used for the prediction of the frequency in blood after birth.