Amniocentesis is a procedure that removes a sample of fluid from the amniotic sac for analysis. The amniotic sac is in the uterine cavity and is the fluid-filled structure inside the pregnant uterus that the baby lives in. The fluid in this sac is referred to as amniotic fluid and it contains fetal cells, proteins, minerals and other compounds that can be tested and it is made up primarily of fetal urine.

An amniocentesis is most often done between 16-20 weeks of pregnancy (gestation) but can be performed after 20 weeks as well. During amniocentesis, fluid is removed by placing a long needle through the abdominal wall into amniotic sac. Sometimes, the woman's skin is injected first with a local anesthetic, but this is not usually necessary. The amniocentesis needle is typically guided into the sac with the help of ultrasound imaging performed either prior to or during the procedure. Once the needle is in the sac, a syringe is used to withdraw the clear amber-colored amniotic fluid. The amount of fluid taken out of the sac depends upon the age of the fetus and the reason for the testing.

The amniotic fluid is then sent to a lab and can be used for a large number of tests or studies. The fluid can be used to evaluate fetal lung maturity, to test for specific proteins that may be evidence of spina bifida (a birth defect in spinal cord development) or other neural-tube defects, to determine the presence of infection, or used for a genetic evaluation or chromosome analysis.

Chromosomes are structures that contain all of the genetic information in our cells. The amniotic fluid contains numerous free-floating fetal cells that can be grown in a laboratory. When these cells multiply and reach a certain number, their chromosomes are extracted and analyzed. It takes about two weeks to perform chromosome analysis. Amniocentesis is more than 99 percent accurate in diagnosing chromosomal conditions and disorders such as Down syndrome (trisomy 21), trisomy 13, trisomy 18, and sex chromosome abnormalities (such as Turner syndrome and Klinefelter syndrome).

There are several hundred different genetic disorders and, while this test is not used to look for all of them, it can be used to look for a number of disorders such as cystic fibrosis, sickle cell disease, and Tay-Sachs disease. If your baby is at increased risk for one or more of these disorders, in most cases amniocentesis can usually tell you whether he has the disease.

Amniocentesis does not detect birth defects such as heart malformations or a cleft lip or palate. However, many structural defects can be picked up on the second-trimester ultrasound that's routinely done for every woman having an amnio.

For most patients, amniocentesis is a fairly quick and comfortable procedure. Some women may experience some uterine cramping or a feeling of light headedness. This usually resolves relatively quickly and women are encouraged to drink 8-10 glasses of water and to rest.

Data obtained from amniotic fluid can help women and families make informed decisions regarding their pregnancies and babies.

Who Should Consider Having An Amniocentesis?

Women over the age of 35 have an increased chance of having a baby with chromosomal abnormalities. Because of this, the current recommendation is that women who will be age 35 or older at the time of delivery should consider having an amniocentesis for fetal chromosome testing.

Chromosomal abnormalities in the fetus can lead to mental retardation or other birth defects. The possibility of having a baby with a chromosomal defect increases with the age of the mother. At age 35 the chance is about 1 in 178, by age 40, the rate increases to 1 in 63, and by age 48, the ratio is 1 in 8. For women younger than 35, the risk of complications as a result of amniocentesis may outweigh the possibility of having a baby with a chromosomal abnormality. Less than 1 in 270 women who have an amniocentesis will have a complication from this procedure.

Amniocentesis may be recommended if there is a family history of chromosomal problems or a history of prior births in which a chromosomal problem was found, However, the exact benefit of amniocentesis in these situations is uncertain. Because many chromosomal defects may not reappear in subsequent pregnancies, genetic counseling can help a woman decide whether amniocentesis would be of value in her specific situation.

Women who have a positive screening result for chromosomal or neural tube defects should also receive genetic counseling and consider further testing.

It is important to us that we give you have all the information you need to make the best decisions possible when considering whether or not to have an amniocentesis or chorionic villus sampling.