Confined Placental Mosaicism

Posted On: July 13, 2022 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 chorionic villous (CVS) testing or other genetic screenings. Confined placental mosaicism is a rare and often inconsequential condition that only affects about 1-2% of pregnancies.

What is Confined Placental Mosaicism?

Confined placental mosaicism occurs when the tissue of the placenta has an abnormal number of chromosomes, but the fetus itself has a normal number of chromosomes. In a majority of cases, confined placental mosaicism does not cause any health complications for either the pregnancy or the baby. Your doctor may recommend additional testing, such as an amniocentesis, to be sure that this is the case. Confined placental mosaicism is rare, occurring in about 1-2% of all pregnancies.

What is CVS Testing?

CVS testing involves taking a sample of tissue from the placenta in order to screen for genetic or chromosomal abnormalities. It is usually conducted during the first trimester between 11 and 13 weeks of pregnancy. CVS testing is sometimes recommended for patients who fall into the following categories:

  • Family history of genetic diseases, chromosomal abnormalities, or metabolic disorders
  • Increased risk for genetic abnormality based on screening tests (such as a nuchal translucency screening, first-trimester blood test, or noninvasive prenatal testing)
  • Abnormal or concerning ultrasound finding
  • Desire to have as much genetic information as possible about the fetus. For this reason, CVS should be offered to ALL pregnant women, regardless of maternal age, history, or screening test results.
  • Some doctors recommend CVS for all women age 35 and older.

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. Transcervical CVS tends to be less painful, but more likely to cause bleeding.

Transabdominal CVS tends to be more painful; (as it involves a needle), but rarely causes bleeding. Both are equally safe and accurate. In the United States, more than 95% of CVS’ are done transabdominally. For both methods, ultrasound imaging is used to guide the testing and locate the placenta to gather the sample. Both methods of CVS testing usually take about 30 minutes to complete.

How is Confined Placental Mosaicism Diagnosed?

When women undergo chorionic villus sampling (CVS), the sample is analyzed to examine the fetus’ karyotype, which is a visualization of the number of chromosomes present. Usually, the karyotype results come back normal, with 46 normal–appearing chromosomes. Sometimes, unfortunately, the karyotype is abnormal, with extra or missing chromosomes. 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 fetus, meaning 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 abnormal number of chromosomes (aneuploidy) that was present in the blastocyst originally, but was corrected in the fetus through a helpful mutation. The cells of the fetus separate from the cells that will become the placenta mere days after conception, meaning the two tissues are very closely linked genetically, but not identical. This is what allows for CPM to develop in the first place. The later in development a genetic mistake occurs in either the placenta or the fetus, the better the outcome and the less mosaic the tissue will be.

In any 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 normal 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.

Another test with limited usefulness that may be performed before CVS or amniocentesis is cell-free DNA (cfDNA) analysis, which is a more simple procedure that can help detect genetic issues without the invasive risks of the other procedures. Although a cfDNA test can accurately detect markers for genetic abnormalities like trisomy 21 (Down syndrome), that does not mean that a positive result for abnormal chromosomes is guaranteed to be correct. These false positives could indicate a condition like CPM, but would still warrant further testing to be sure.

What Do Confined Placental Mosaicism Diagnoses Mean?

Often, a confined placental mosaicism diagnosis will prompt your doctor to suggest further testing to ensure your baby’s health and screen for any potentially harmful abnormalities. This can mean genetic counseling or an amniocentesis to test amniotic fluid among other types of screening. Key factors we look for during further testing is the degree to which the placenta or fetus has mosaicism (up to 50% of sampled cells), as well as the genetic mutation that is causing this circumstance.

When confined placental mosaicism is diagnosed, there are several other considerations that our maternal fetal medicine specialists weigh when creating a treatment plan:

  • Although a normal amniocentesis result generally suggests a normal fetal karyotype, there is still a small chance that an actual fetal mosaicism is present, meaning that 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.

Mosaicism in Adults

When geneticists use the term “mosaicism,” they refer to something that can be life-threatening in a developing fetus, but also a benign situation for regular healthy people to live with. Mosaicism simply means that not every single cell in a person’s body has the correct number of chromosomes. This can affect as much as 50% of the cells in the body, and, depending on the severity of the mistake in DNA transcription, can lead to diagnoses like Mosaic Down syndrome or Turner syndrome in the affected cells.

There is no way to predict mosaicism, but it can be detected through chorionic villus sampling during pregnancy. People who do not have severe mosaicism or whose DNA is only altered slightly may never know they have mosaicism.

Schedule an Appointment

To meet with an OB/GYN or Maternal Fetal Medicine Specialist to learn more about confined placental mosaicism and your testing options, call Carnegie Imaging or contact us online. It’s important to remember that an abnormal test result is not the same as a guaranteed diagnosis. When thinking about CVS and CPM, these are all low-risk, rare complications that can allow you to have a healthy baby at the planned time. Our maternal fetal medicine specialist and genetic counselor work hard to explain these results in context to help minimize your anxiety during pregnancy.

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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