When performing genetic analysis on products of conception, an amniocentesis sample or villous sampling, it is always possible that the findings may represent contamination with maternal tissue rather than reflect fetal findings.
Maternal cell contamination may occur:
(1) if maternal tissue is sampled rather than fetal
(2) fetal tissue is nonviable
Maternal cell contamination should be suspected if:
(1) the fetus is necrotic
(2) the results of culture are XX but the phenotype is male
Specimens required:
(1) sample submitted for genetic analysis supposedly from fetus
(2) blood sample from the mother
Testing consists of determining the 2 alleles at each of one or more polymorphic loci.
If the maternal alleles match the sample alleles exactly, then there is maternal cell contamination.
If the sample shows one maternal and one non-maternal allele, then the sample represents fetal tissue.
The number of loci to test may vary between laboratory. Antoniadi et al found that often a single locus was sufficient.
Antoniadi et al tested a specimen from the purported father in addition to the 2 above. This helped confirm paternity.
False negatives may occur if maternal cell contamination is very low.
False positives may occur if uniparent disomy occurs (fetus inherits both chromosomes from one parent and none from the other). This would argue for testing for more than one locus, preferrably from different chromosomes.
