Amniocentesis To Collect Amniotic Fluid and Cells

In an amniocentesis (Greek: amnion = little lamb, kentesis = puncture), the amniotic sac-commonly called the bag of waters-is punctured to collect amniotic fluid and cells shed into it by the fetus. Amniocentesis for fetal diagnosis can be performed between 14 and 17 weeks of pregnancy. Recently, some physicians experimented with performing amniocentesis earlier, but a study of more than 4,000 women in Canada, reported in 1998, showed that amniocentesis at 11 or 12 weeks was associated with a rate of fetal loss higher than both later amniocentesis and CVS. In addition, the babies born to women after early amniocentesis showed an increase of clubfoot.Amniocentesis (amnio) is always accompanied by sonography to localize the fetus and the placenta and to find a pool of amniotic fluid . The pregnant woman lies on her back, her abdomen is cleansed with a sterile solution, and a small needle is inserted through her skin and muscles into the fluid. Some physicians inject a small amount of local anesthetic under the skin. A tiny needle is used for this and it feels like a pinprick. Other physicians feel that this is not necessary and that it limits their ability to change the area of injection if the fetus moves just before they are about to insert the amniocentesis needle. For this and other procedures, you may want to discuss in advance whether or not you have a preference for local anesthesia, or whether it is the physician’s practice to inject it or not, so you are prepared.

The needle used for the amniocentesis is about the same diameter as a needle used to draw blood from an arm vein, but is longer, since it must go through the mother’s skin and abdominal and uterine muscles to reach the fluid. For most women, the procedure causes minimal pain. The needle’s entry is felt as a slight pinprick sensation, but there may be pressure as the needle moves into the uterus. The uterus lies against the mother’s abdominal wall, so there is no risk of injuring other abdominal organs.

Once a suitable fluid sample is obtained, usually about 30 cc or 2 tablespoons, the needle is withdrawn and a small bandage is placed over the puncture site. ‘Withdrawing the fluid takes less than a minute. Shortly after the test is completed, most women can get up and go about their business. There may be soreness at the site of the needle stick for a day or two. Generally, women are advised not to lift anything heavy for 2 days. Mter an amniocentesis, you must call your physician or mid­wife if you have any vaginal leakage of fluid, fever, severe cramps, or bleeding. Slight cramping might occur for the first day and is normal. A small amount of fluid leakage from the vagina is often normal as well, but you still should report it so it can be evaluated and followed if necessary.

Once the amniocentesis is completed, the fluid is placed into sterile tubes and sent to a laboratory. There, it is placed onto a culture medium that allows the cells to grow. Cell growth takes about 1 to 2 weeks. The cells are then “harvested” and treated with chemicals. These chemicals stop the process of cell division at the stage when the chromosomes can be seen most clearly and cause the chromosomes to swell and spread apart. The cells are stained and viewed under a high-power microscope. The chromosomes are photographed and mapped for analysis. The analysis determines whether there are errors in the numbers of chromosomes (aneuploidy), such as extra chromosomes or missing chromosomes, and whether there are errors in the structure of chromosomes, such as missing pieces of chromosomes or mixing up of chromosomes (translocations).

The amniotic fluid is assessed for evidence of markers of disease, such as elevated alphafetoprotein, as described in the section on screening of maternal blood. Direct analysis of alphafetoprotein in amniotic fluid is considered to be 99 percent accurate in diagnosing neural tube or other defects. Fresh amniotic fluid and the liquid culture medium also can be examined for the presence of enzymes and other biochemicals whose presence indicates various genetic disorders such as Tay-Sachs disease, cystic fibrosis, combined immune deficiency, sickle-cell anemia, and thalessemia major, as well as other less common diseases.

Amniocentesis has been employed for fetal treatments to administer medication to the fetus. It is a part of the process of in utero fetal blood transfusion. Amniocentesis is also used in late pregnancy to assess the amount of a lipid (fatty) chemical which gives an estimate of fetal lung maturity. This is one of the possible tests performed when there is a question as to whether a baby who is not growing properly would thrive better if left to develop to maturity inside the mother or if delivered and cared for in a neonatal intensive care unit. .

A recently developed technique called FISH may speed diagnosis in some cases. This technique requires knowing the DNA sequence where a specific gene is located so that a probe for that gene can be applied to allow visualization of the part of the chromosome of interest. It can also be used to demonstrate abnormal chromosome numbers. FISH has two advantages over currently used procedures, it is accurate when there are only a few cells obtained and it does not require capturing the cells at the time of division. Because of the first advantage, FISH might be used eventually on the fetal cells that find their way into the maternal blood, in the future perhaps decreasing the need for some amniocenteses. Because of the second advantage, results from CVS and amniocentesis could be obtained without the delay of growing the cells over time.