Common Prenatal Diagnosis Procedures, Amniocentesis or Chorionic Villus Sampling, may be appropriate for women who are at an increased genetic risk for one of several reasons. The most common ones include:
- Mother age 35 or older on delivery date
- Positive 1st Trimester Screen or Quad Screen
- Concerning findings noted by prenatal ultrasound
- Both parents carry a recessive gene (cystic fibrosis, sickle cell, etc.)
- One parent carries a chromosomal rearrangement
- Mother carries Fragile X or another X chromosome disease gene
- Prior child with a chromosome abnormality
Amniocentesis or “amnio” usually is done after 16 weeks of pregnancy. The procedure involves obtaining amniotic fluid from around the fetus. This fluid contains fetal cells that can be used for genetic tests. The level of AFP in the fluid is also a very good test to check for neural tube defects. Amniocentesis is done under ultrasound guidance through the mother’s abdomen using a very thin needle. Though women are usually nervous before about having amniocentesis, most say afterwards that the procedure is quick, easy and surprisingly painless! As with any medical procedure, there is a small risk. The risks of amniocentesis include bleeding, infection, leakage of fluid, and miscarriage. The chance of miscarriage from amniocentesis is no greater than 1 in 500. The other complications are very uncommon.
Chorionic Villus Sampling (CVS)
The chorionic villus sampling (CVS) procedure involves obtaining a sample of fetal cells (chorionic villi) from the placenta. It is performed between 10 and 13 weeks of pregnancy. The procedure can be done through the cervix with a thin plastic tube, or it can be done through the mother’s abdomen with a needle, depending on the location of the placenta. It is done under ultrasound guidance. An advantage of CVS is the ability to obtain genetic information about the fetus in the first trimester of pregnancy. CVS cannot provide any information about neural tube defects, so if you are at risk for a neural tube defect, amniocentesis, rather than CVS will be offered. The risks of CVS include bleeding, infection, and miscarriage. The miscarriage rate from CVS is less than 1%. Though CVS is an accurate test, it occasionally yields false abnormal results that require amniocentesis for interpretation.
Both amniocentesis and CVS involve obtaining fetal cells, growing them in the laboratory, and performing the necessary genetic tests. Because the cells must be grown and even the most common tests are complex, results typically take 10-14 days, but may take longer if special or unusual tests are required.