If you receive abnormal results from the first or second trimester screening panels, your doctor may recommend that you speak to a genetic counselor, who will be able to help you better understand what the results mean, the best way to establish an accurate diagnosis, what the procedures entail, what the risks are, and what your options will be once a diagnosis is established. The two most common invasive fetal tests are amniocentesis and chorionic villus sampling.
Amniocentesis can detect chromosomal disorders like Down’s Syndrome (Trisomy 21), neural tube defects such as spina bifida, and other genetic disorders like cystic fibrosis and Tay Sach’s disease. The DNA obtained from amniocentesis can also be compared to the potential father’s DNA to establish paternity. The test is usually done between 14 and 20 weeks, though it can be performed earlier or later than this depending on the reason for testing. The procedure involves inserting a long but very thin needle through the abdominal and uterine wall and drawing a small amount of amniotic fluid out. The fluid contains cells from the fetus and is sent to a lab for analysis. The whole procedure takes place under the guidance of ultrasound to minimize the risk of hitting the fetus or placenta with the needle. It is a commonly performed and fairly safe procedure; however, it does increase the risk for miscarriage slightly if an infection develops in the uterus, labor starts, or the membranes rupture prematurely. The risk is documented as somewhere between 1 in 200 and 1 in 400 depending on the facility. More common side effects include mild cramps and light leakage of amniotic fluid for 24 hours or less.
Chorionic Villus Sampling (CVS) can also detect chromosomal and genetic disorders as well as establish paternity. It cannot, however, test for neural tube defects. This test is usually performed earlier than amniocentesis (between 10 and 14 weeks) in women who are at high risk for genetic or chromosomal disorders based on age, race, or family history, as well as in women who had an abnormal result on their first trimester nuchal translucency and lab testing. The procedure involves removing chorionic villi cells from the placenta. These cells contain the same DNA as the fetus and can therefore be analyzed to determine if the fetus has any genetic or chromosomal abnormalities. The most common way to obtain these cells is by guiding a thin catheter through the cervix and into the placenta under the guidance of ultrasound. However, the needle can also be guided through the abdominal wall and into the placenta, which is recommended for women with uterine fibroids or a tilted uterus. Like amniocentesis, CVS is a common and fairly safe procedure but does carry a risk for miscarriage, occurring in 1 out of 100 procedures. It is not recommended for women with an active STD infection, who are carrying multiples, or who have had vaginal bleeding during pregnancy. Side effects include infection, spotting, and cramping.
Invasive tests are relatively safe but do pose a slight risk for miscarriage. It is important to discuss these procedures with your doctor before deciding whether you do or do not want to have them performed. You should consider the risks, the potential benefits, and what your options will be if you get a positive result. Will the information that you obtain from the test change your decision-making process or medical care? If you still feel uncertain after discussing with your doctor, consider making an appointment with a genetic counselor, who might be able to make things more clear. It is a tough choice, where the potential diagnosis, your values, and life experiences all come into play, but it is ultimately your decision to make.