DNA analysis of first-trimester chorionic villous biopsies: test for maternal contamination.

We investigated the reliability of chorionic villous biopsy as a method to obtain tissues reflecting the genetic constitution of the embryo. In 12 pregnancies before elective termination, we searched for detectable maternal tissue after careful dissection of villi from small 2-5-mg specimens that yielded 7 micrograms of DNA per mg tissue. In Southern blotting experiments (1-2 micrograms DNA per lane), restriction fragment length polymorphisms (RFLPs) at an autosomal (D14S1) and a sex chromosomal (DXYS1) locus allowed recognition of maternally and embryonically derived alleles. Pure villi were obtained in six of the seven informative cases. One biopsy was not dissected satisfactorily; a mixture of embryonic and maternal DNA was found. Nonvillous tissues were mostly maternally derived in eight informative cases. Sex determination by molecular analysis (alleles at the DXYS1 locus) agreed with the karyotypes of uncultured or cultured villi. In one continuing pregnancy, distinct RFLPs indicated maternal inheritance of the alpha-thalassemia 1 trait in a female embryo without detectable maternal contamination. Reliable prenatal diagnosis depends on the specimen's purity. Maternal contamination can be evaluated by DNA analyses.