Prenatal diagnosis for fragile X syndrome was performed in 34 pregnancies of 33 known carriers, on 22 chorionic villus samples (CVS), and 15 amniocentesis samples. Fetal and maternal DNA were analyzed by the EagI/EcoRI Southern blot of Rousseau et al. [1991: N Engl J Med 325:1673-1681], with detection of full mutations ensured by a second loading with brief electrophoresis. As a supplemental assay for full mutations, cytogenetic induction was performed in 20 cases. Positive cytogenetic results were helpful in confirming full mutations in CVS cases where the fetal DNA was intermediate in appearance, between a large premutation and a small full mutation. Of 8 mothers with full mutations, the fetal results were 5 full, 2 normal, and 1 premutation (whose mother was a full/pre compound heterozygote). Of 26 mothers with premutations, the fetal results were 5 full, 13 normal, 7 premutation, and 1 uninterpretable (maternal contamination). Maternal premutations were sized in kb by Southern blot and in CGG repeat number by PCR; the predicted correlation between maternal length and penetrance was seen. Follow-up studies include 3 full mutations and 2 premutations confirmed by DNA analysis at birth. Maternal contamination of CVS samples was encountered in 3 of 22 cases, illustrating the value of EagI in detecting maternal (lyonized) chromosomes.

The objective of this study was to evaluate the accuracy of a DNA-based testing methodology in determining the RhD genotypes of fetuses at risk for RhD hemolytic disease. We designed a multiplex polymerase chain reaction-based test based on recent RhD and RhCE sequence information. To improve the accuracy of the results, two different portions of the RhD gene were examined. Deoxyribonucleic acid was extracted from fetal specimens, portions of the RhD gene were amplified by the polymerase chain reaction, and the amplified product was run on a polyacrylamide gel to look for the presence or absence of the RhD gene. We tested 67 amniotic fluid and two chorionic villus specimens to determine the fetal RhD genotype in pregnancies at risk for RhD hemolytic disease. Forty-seven of the 69 specimens were determined to be Rh-positive, and 22 were Rh-negative. Fifty of the 69 fetal specimens--31 Rh-positive and 19 Rh-negative--were serotyped at birth. In all 50, there was complete correlation between the DNA analysis and the serotyping results. RhD gene analysis is a rapid and reliable method that provides an accurate fetal genotype to aid in the prenatal care of RhD-alloimmunized women.