Amniocentesis, CVS, and Cordocentesis

1. Amniocentesis

What is Amniocentesis?

Amniocentesis is a prenatal diagnostic test that involves the removal of a small amount of amniotic fluid (the fluid surrounding the fetus in the womb) from the uterus for analysis. This fluid contains fetal cells and proteins that can be tested for genetic, chromosomal, and metabolic conditions.

When is Amniocentesis Performed?

• Timing: Typically done between 15 and 20 weeks of pregnancy, although it can sometimes be done later.
• Indications: Commonly offered when there are increased risks for genetic or chromosomal disorders, such as:
  • Advanced maternal age (e.g., 35 years or older)
  • Abnormal results from screening tests (e.g., NIPT, ultrasound)
  • Family history of genetic disorders
  • Previous child with a genetic disorder
  • Maternal conditions such as diabetes, which increase the risk of certain birth defects

Procedure:

1. Ultrasound Guidance: The procedure is performed under ultrasound guidance to ensure the needle avoids the baby, placenta, and umbilical cord.
2. Needle Insertion: A fine needle is inserted through the abdomen into the uterus, and a small amount of amniotic fluid is withdrawn (usually around 15–20 milliliters).
3. Fluid Analysis: The sample is then sent to the laboratory for genetic testing, chromosomal analysis, and/or testing for infections and metabolic disorders.

Tests Performed on Amniotic Fluid:

• Karyotyping: To check for chromosomal abnormalities like Down syndrome (Trisomy 21), Trisomy 18, and Trisomy 13.
• Fetal DNA analysis: To detect specific genetic disorders (e.g., cystic fibrosis, sickle cell anemia).
• Neural Tube Defects: Amniotic fluid can also be tested for elevated levels of alpha-fetoprotein (AFP), which can indicate the presence of neural tube defects like spina bifida or anencephaly.
• Fetal lung maturity: In cases of premature labor, amniocentesis can be used to assess lung maturity.
• Infections: The fluid may be tested for infections like toxoplasmosis, syphilis, or cytomegalovirus.

Risks and Complications:

• Miscarriage: The risk of miscarriage from amniocentesis is about 0.1-0.3%.
• Infection: There is a slight risk of infection from the procedure.
• Injury: Rarely, the needle can cause injury to the fetus or the placenta.
• Leakage of Amniotic Fluid: A small amount of fluid may leak from the site of the needle insertion, though this is typically harmless.
• Preterm Labor: In rare cases, amniocentesis may trigger early labor.

2. Chorionic Villus Sampling (CVS)

What is CVS?

Chorionic Villus Sampling (CVS) is a prenatal test that involves taking a sample of tissue from the chorionic villi, which are tiny finger-like projections from the placenta that share the fetus’s genetic makeup. This tissue is analyzed to detect genetic conditions and chromosomal abnormalities.

When is CVS Performed?

• Timing: CVS is usually done between 10 and 13 weeks of pregnancy.
• Indications: CVS is often recommended when there is a high risk of genetic disorders or chromosomal abnormalities in the fetus, such as:
  • Advanced maternal age
  • Abnormal results from screening tests (e.g., NIPT, ultrasound)
  • Family history of genetic conditions
  • Previous pregnancy with a chromosomal abnormality

Procedure:

There are two main approaches to performing CVS:

1. Transcervical CVS: A thin tube (catheter) is inserted through the cervix and into the placenta to obtain the sample of chorionic villi.
2. Transabdominal CVS: A needle is inserted through the abdomen into the placenta to obtain the sample.

Both methods are performed under ultrasound guidance to ensure proper placement and minimize risks.

Tests Performed on Chorionic Villi Tissue:

• Karyotyping: For detecting chromosomal abnormalities (e.g., Down syndrome, Trisomy 18).
• DNA testing: To check for specific genetic disorders like cystic fibrosis, sickle cell anemia, or muscular dystrophy.
• Sex chromosome analysis: Can be used to determine fetal sex and check for conditions like Turner syndrome or Klinefelter syndrome.

Risks and Complications:

• Miscarriage: The risk of miscarriage after CVS is about 0.5-1%, which is slightly higher than amniocentesis.
• Infection: As with amniocentesis, there is a small risk of infection.
• Injury: There is a very small risk of injury to the fetus, placenta, or cervix.
• Bleeding or Spotting: Some bleeding or spotting may occur after the procedure, especially with transcervical CVS.

3. Cordocentesis (Percutaneous Umbilical Blood Sampling – PUBS)

What is Cordocentesis?

Cordocentesis, or Percutaneous Umbilical Blood Sampling (PUBS), is a prenatal test where a sample of fetal blood is taken from the umbilical cord using a needle inserted through the abdominal wall and into the uterus. This test is most often performed when a more detailed analysis of fetal blood is needed, particularly in late pregnancy.

When is Cordocentesis Performed?

• Timing: Usually done after 18 weeks of gestation, but often performed between 20 and 24 weeks.
• Indications: It is typically done in cases where:
  • There is a need for fetal blood analysis to confirm genetic conditions or assess fetal anemia (due to Rh incompatibility or other causes).
  • The fetus is suspected to have infections (e.g., parvovirus B19).
  • There is a need to measure fetal oxygen levels, acidity, or metabolic status.
  • Genetic disorders suspected after CVS or amniocentesis.
  • To assess fetal platelet counts or diagnose certain infections.

Procedure:

• Ultrasound Guidance: The procedure is performed under ultrasound guidance.
• Needle Insertion: A fine needle is inserted through the abdomen into the umbilical cord to withdraw a small sample of fetal blood.
• The blood sample is analyzed for genetic or chromosomal conditions, infection, or blood disorders.

Tests Performed on Fetal Blood:

• Karyotyping: For detecting chromosomal abnormalities.
• Genetic

• testing: To diagnose conditions like thalassemia, sickle cell anemia, and other inherited blood disorders.
• Infection Testing: For fetal infections like parvovirus B19 or cytomegalovirus.
• Fetal Blood Counts: To assess for fetal anemia or platelet disorders.

Risks and Complications:

• Miscarriage: The risk of miscarriage after cordocentesis is higher than with amniocentesis or CVS, estimated at 1-2%.
• Infection: There is a risk of introducing infection into the uterus or umbilical cord.
• Injury to the Fetus: As with other invasive tests, there is a slight risk of injury to the fetus, especially to the umbilical cord.
• Preterm Labor: The procedure may trigger early labor in rare cases.

Comparison of Amniocentesis, CVS, and Cordocentesis

Summary

• Amniocentesis: A test performed typically in the second trimester (15-20 weeks) to analyze amniotic fluid for genetic and chromosomal abnormalities, neural tube defects, and other fetal conditions.
• Chorionic Villus Sampling (CVS): A test usually done in the first trimester (10-13 weeks) to take a sample from the placenta for genetic testing and chromosomal analysis.
• Cordocentesis (PUBS): Performed in late pregnancy (20-24 weeks), involving the extraction of fetal blood from the umbilical cord to test for conditions like anemia, blood disorders, and infections.