Confined Placental Mosaicism

Posted On: June 19, 2020 By Nathan D. Fox, MD

Confined placental mosaicism (CPM) is a condition in which the placenta and the fetus each have a different chromosomal makeup. This condition is sometimes discovered via CVS testing. Confined placental mosaicism is a rare and often inconsequential condition which only affects about 1-2% of pregnancies.  

What is CVS Testing? 

CVS, of chorionic villus sampling, testing involves taking a sample of tissue from the placenta in order to screen for genetic or chromosomal abnormalities. It is usually conducted between 10 and 12 weeks of pregnancy, and is sometimes recommended for patients with a family history of genetic diseases, chromosomal abnormalities, or metabolic disorders, patients over 35 years old or at increased risk for a agenetic abnormality on screening testsor those who have had an abnormal or concerning ultrasound finding. The procedure can be completed through a transcervical method, in which a catheter is inserted through the cervix to retrieve placental tissue, or using a transabdominal method, in which a needle is inserted through the abdomen and uterus to retrieve the placental tissue.  

How is Confined Placental Mosaicism Diagnosed? 

When women undergo chorionic villus sampling (CVS), the sample is analyzed to examine the fetus’ karyotype, or a visualization of the number of chromosomes present. Usually, the karyotype results come back normal, with 46 normalappearing chromosomes. Sometimes, unfortunately, the karyotype is abnormal, with 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 (the structure that forms early in development which later becomes the embryo). However, approximately 1 to 2% of all placental karyotypes do not match the fetusmeaning that the placenta could have an abnormal or mosaic karyotype, but the fetus does not. 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 (abnormal number of chromosomes in the cell) often not compatible with life. For example, if the fetus appears to be healthy and abnormal but the CVS test shows that the placenta has a karyotype that would usually be associated with a miscarriage or stillbirth, a doctor may assume that this is a result of confined placental mosaicism.  

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. 

What Do Confined Placental Mosaicism Diagnoses Mean? 

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 on anatomic ultrasound. 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 recommended as further 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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