Amniocentesis

Definition

Amniocentesis is a simple medical procedure used to obtain small samples of the amniotic fluid surrounding the fetus.

Description

In the first half of the pregnancy (between the 14th and 18th week) the doctor may perform an amniocentesis when:

• The age of the mother is 35 years of age or older. The risk of bearing children with chromosomal birth defects increases as a woman ages, thus if a woman will be 35 or older at the time of delivery, most physicians offer the option of prenatal testing for chromosomal disorders. Among the most common of these disorders is Down syndrome, a combination of mental and physical abnormalities caused by the presence of an extra chromosome.

• A previous child or pregnancy resulted in a birth defect. If a couple already has a child (or pregnancy) diagnosed with a chromosomal abnormality, a biochemical birth defect, or a neural tube defect, the couple may be offered prenatal testing during subsequent pregnancies.

• Family history shows an increased risk of inheriting a genetic disorder. Couples without a previously affected child may also be offered prenatal testing if their family medical histories indicate their children may be at increased risk of inheriting a genetic disorder.

• One or both prospective parents may be "carriers" of a disorder, or a disorder may "run in the family." Prenatal testing would be done only if the suspected condition can be diagnosed prenatally.

• There is suspected neural tube defects. These defects of the spine and brain, including spina bifida and anencephaly, can be diagnosed by measuring the level of alphafetoprotein (AFP) in the amniotic fluid. Amniocentesis to measure AFP is offered if there is a family history of neural tube defects, or if earlier screening tests of AFP in the mother's blood indicate that the pregnancy is at increased risk.

• The doctor wants to assess fetal lung maturity. Fetal lung assessment is important if the mother needs to deliver the baby early. By testing the amniotic fluid doctors can tell whether the baby's lung's are developed enough to breathe on their own.

• Detection of Rh disease is needed. Rh disease causes antibodies in the mother's blood to attack fetal blood cells and an amniocentesis detects the disease and enables the doctors to take appropriate measures to reduce complications.

Procedure

Ultrasound is used to see exactly where the fetus and placenta are located, which allows the doctor to choose the safest spot for inserting the amniocentesis needle.

The abdomen is cleansed and sometimes a local anesthetic is injected just beneath the skin. A thin needle is inserted through the abdomen and into the uterus, where one to two tablespoons of amniotic fluid are withdrawn.

After the sample is taken, an ultrasound check of the baby's heart movements is done before the woman leaves the examination room.

A few women feel cramping when the needle enters the uterus or pressure during the few minutes while the fluid is being withdrawn. Many doctors recommend that women rest for several hours after amniocentesis, avoiding physical stresses such as lifting and prolonged standing. If the cramping continues, or there is leakage or discomfort around the needle site, call the doctor.

Living cells from the woman's fluid sample are grown in the laboratory, and then tested for chromosomal abnormalities or various genetic birth defects. Results of laboratory studies of cells after amniocentesis usually take from one to three weeks and are 99 percent accurate in diagnosing and ruling out chromosomal abnormalities.

Amniocentesis is not routinely offered during pregnancy because the test itself carries a small risk of infection, miscarriage (less than 1 in every 200 women), fetal or placental injury, bleeding, premature labor and Rh sensitization.

Questions to Ask Your Doctor

Should an amniocentesis be done?

Is there a possibility the baby will have birth defects?

When should the amniocentesis be done?

How will the test be done?

What will the amniocentesis show?

What are the possible outcomes?