Prenatal genetic testing performed on samples obtained through chorionic villus sampling (CVS) or amniocentesis analyzes cells originating from the developing pregnancy. The reliability of any molecular or chromosomal result depends on sample purity. Maternal Cell Contamination (MCC) describes a scenario where a prenatal sample contains a mixture of maternal cells alongside the fetal or placental tissue intended for analysis.
MCC is a well-documented occurrence in prenatal diagnostics. However, because it directly impacts result accuracy, understanding why MCC occurs and how laboratories screen for it helps families properly interpret diagnostic reports.
Why It Happens
During invasive prenatal procedures, maternal cells can inadvertently mix with the fetal sample:
- Chorionic Villus Sampling (CVS): Placental tissue is aspirated transcervically or transabdominally. Because maternal decidual tissue lies directly adjacent to the chorionic villi, small amounts of maternal tissue can be collected alongside the placental sample.
- Amniocentesis: Amniotic fluid containing shed fetal cells is collected via an ultrasound-guided needle inserted through the abdominal wall. Maternal blood or skin cells can be introduced at the needle entry point during collection.
The proportion of maternal cells varies per sample. While a minor trace of maternal cells in a large sample may not interfere with analysis, a high proportion of maternal cells creates a substantial risk of analyzing the mother’s DNA rather than the fetus’s.
Why It Matters for Diagnostic Accuracy
The significance of maternal cell contamination depends on the type of test being performed and the proportion of contaminating cells in the sample. For chromosomal analysis, a small amount of contamination may not affect the result because the fetal cells are still the majority being analyzed. For molecular testing looking for a specific genetic variant, even a lower level of contamination can be problematic because the normal maternal cells could mask a fetal abnormality, potentially producing a false normal result.
The maternal cell contamination test uses genetic markers to distinguish between the DNA profile of the pregnant person and the DNA profile in the prenatal sample. If the sample shows only or predominantly the maternal profile, the laboratory identifies contamination and reports this finding.
What Happens When Contamination Is Found
When the laboratory detects contamination at a level that affects the reliability of the result, the options depend on how significant the contamination is and what kind of testing was being done. For cell culture-based testing, it may be possible to allow the culture to grow further so that fetal cells can outgrow the maternal ones, after which the analysis can be repeated. For direct molecular testing, a repeat procedure on a fresh sample may be recommended.
MedGenome includes maternal cell contamination assessment as a quality assurance step in its prenatal genetic testing processes, which means samples are checked for this issue routinely rather than only when a result looks suspicious. A result that has passed this quality check provides a higher level of confidence than one reported without it.
