Perinatal Procedures
What You Need to Know
What is an Amniocentesis and CVS?
Amniocentesis is a procedure used to draw a sample of the amniotic fluid that surrounds the baby in the womb. This fluid contains cells that have been sloughed off of the developing fetus.
Amniocentesis is most often used to detect genetic abnormalities, though the results can also reveal the baby's sex, gauge lung maturity (if done close to term), detect amniotic fluid infections, or diagnose other pregnancy related problems.
Amniocentesis can diagnose or rule out many possible fetal abnormalities. Most often, it's used to spot common genetic defects (such as Down syndrome) and neural tube defects (such as spina bifida). Amniocentesis is usually performed between 16 to 22 weeks gestation, although it can be done as early as 13 - 14 weeks and beyond 22 weeks gestation.
Chorionic villas sampling (CVS), a similar test is sometimes used to detect many of the same abnormalities. Instead of amniotic fluid, CVS examines placental cells, which are genetically identical to the fetus. Since CVS can be done earlier in the pregnancy (~ 11 – 12 weeks), and since results are available within 48 hours, it may be preferable for women who need earlier answers about their baby's health. CVS carries a higher risk of miscarriage than amniocentesis, and may increase the risk of limb deformities if performed too early.
What Are the Risks?
About one woman in every 200-400 women can have a miscarriage as a result of amniocentesis, although recent studies suggest that the rate of miscarriage may be as low as one in 1,000 (0.1%). Amniocentesis done during the first trimester carries a greater risk for miscarriage than amniocentesis done after the 15th week. The risk of miscarriage can be similar to that caused by CVS, which is ~ 1:100. Less than one woman in every 1,000 women develops a uterine infection after amniocentesis.
How Can I Prepare?
Amniocentesis provides diagnostic information about the baby. It not a form of treatment, however can provide information to the mother to make certain decisions about treatment. So before you have the test done, think about how you might use the information it provides. Some birth defects are incompatible with life, others are easily correctable, and still others fall somewhere in between.
Weigh the benefits and risks. If the information provided from either CVS or amniocentesis doesn’t impact on your decision about the pregnancy, the test may add more anxiety than it's worth. On the other hand, knowing your baby has a problem may allow you to prepare better for his or her birth. One study of mothers who continued pregnancies after learning their babies had Down syndrome showed they were much better able to cope than women who discovered at birth that their babies were affected.
If you decide to proceed with the amniocentesis or CVS, there is no specific preparation needed, except to maintain a relaxed & calm disposition leading up to the day of the procedure and thereafter. After either procedure, patients are typically advised to maintain reduced activities and no heavy lifting for 12 – 24 hours. Prolonged bed rest or absence from work is not necessary.
What Will Happen?
First, an ultrasound is done to visualize the position of the fetus and placenta. This allows your doctor to determine the safest place to insert the needle. Guided by ultrasound, the doctor carefully inserts a hollow needle through your abdomen, into the amniotic sac, and draws out about four teaspoons (~20 cc) of amniotic fluid. Your baby quickly replaces any fluid that is lost within 12 - 24 hours. During the course of the procedure the patient can observe her baby’s activity before, during and after for reassurance.
If a local anesthetic is used, you may feel a stinging sensation for a few seconds. You may also feel slight pain or cramping when the doctor inserts the needle and pressure in your lower abdomen when the fluid is withdrawn. Some women report no pain or discomfort, and find the “anticipation” of the procedure the biggest obstacle. After the fluid is removed, the fetal cells are grown in a laboratory and then analyzed. Your doctor will get the complete results within 7 -10 days. On occasion a rapid preliminary test result can be obtained in ~ 24 -48 hours.
Additionally the level of alpha-fetoprotein in the amniotic fluid is measured, which may confer certain risk if abnormally elevated (e.g. risk of neural tube defect). Since alpha-fetoprotein does not need to be cultured in the lab, these results are available right away -- although a potential problem must be confirmed by subsequent genetic analysis of the amniotic fluid.
Who Should Have Amniocentesis?
Amniocentesis is typically offered only to women at increased risk for bearing a child with birth defects. This includes women who:
- Will be 35 or older when they give birth
- Have a screening test or exam result that indicates a possible birth defect or other problem
- Have had birth defects in previous pregnancies
- Have a family history of genetic disorders
If birth defects run in your family, seek genetic counseling before you have amniocentesis - and if possible, before you get pregnant. You and your partner can have certain blood tests to determine your chances of having a baby with a birth defect. Commonly parental carrier testing for such defects such as Cystic Fibrosis or Sickle Cell anemia may preclude the patient from having an amniocentesis or CVS, pending the results.