Confined Placental Mosaicism

Posted On: May 16, 2016 By Nathan D. Fox, MD

During pregnancy, women sometimes undergo chorionic villus sampling (CVS), which is a diagnostic prenatal test used to detect genetic diseases such as Down’s syndrome. Usually, the karyotype results come back normal (46 normal appearing chromosomes), and sometimes, unfortunately, abnormal (an extra or missing chromosome).  However, approximately 1% of the time, the results will show two populations of cells, some normal and some abnormal, which is referred to as Mosaicism.

When performing a CVS, it is assumed that the karyotype of the placenta will be identical to the karyotype of the fetus since the tissues from both originate from the same blastocyst. However, approximately 1 to 2% of all placental karyotypes do not match the fetus. This can be due to a mutation in the placental cell line only, or an aneuploidy (abnormal number of chromosomes) that was present in the blastocyst originally, but was corrected in the fetus through a helpful mutation. In either case, the placenta will have an abnormal mosaic karyotype and the fetus will have a normal karyotype. This condition is referred to as Confined Placental Mosaicism and is suspected when there is a healthy appearing fetus with a CVS result showing an aneuploidy often not compatible with life.

When these differing results occur, amniocentesis is used to obtain the fetal karyotype. Amniocentesis refers to the testing of amniotic fluid to diagnose any chromosomal abnormalities. The cells tested through amniocentesis originate from the fetus directly and not the placenta.  If the test results are normal, confined placental mosaicism is the most likely diagnosis, indicating that the abnormal CVS result was placental only and the fetus is most likely not affected.

When confined placental mosaicism is diagnosed, there are several other considerations:

  • Although a normal amniocentesis result generally suggests a normal fetal karyotype, there is still a small chance that an actual fetal mosaicism is present (some cells in the fetus are normal, such as those found in the amniotic fluid, and other cells are abnormal, such as those in other fetal tissues). Fortunately, this is unlikely especially when there is a normal appearing fetus onanatomicultrasound. Further invasive fetal testing may be recommended to reduce this likelihood.
  • In particular chromosomal abnormalities, confined placental mosaicism is associated with uniparental disomy (UPD). UPD occurs if both fetal copies of a chromosome come from the same parent. Genetic counseling is recommendedasfurther testing to diagnose these conditions.
  • Placental mosaicism is associated with fetal growth restriction, so serial growth ultrasounds are recommended for women with this condition.

Carnegie Imaging for Women blogs are intended for educational purposes only and do not replace certified professional care. Medical conditions vary and change frequently. Please ask your doctor any questions you may have regarding your condition to receive a proper diagnosis or risk analysis. Thank you!

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