Prenatal diagnostic tests
Amniocentesis is a procedure used to diagnose fetal defects in the early second trimester of pregnancy. A sample of the amniotic fluid which surrounds a fetus in the womb is collected through a pregnant woman's abdomen using a needle and syringe. Tests performed on fetal cells found in the sample can revealthe presence of many types of genetic disorders, thus allowing doctors and prospective parents to make important decisions about early treatment and intervention.
Since the mid-1970s, amniocentesis has been used routinely to test for Down syndrome, by far the most common genetic birth defect. By 1997, about 800 different diagnostic tests were available, most of them for hereditary genetic disorders such as Tay-Sachs disease, muscular dystrophy and cystic fibrosis.
Amniocentesis is recommended for women over age 35, or who have already bornechildren with birth defects, or when either of the parents has a family history of a birth defect for which a diagnostic test is available. Another reason for the procedure is to confirm indications of Down syndrome and certain other defects which may have shown up previously during routine blood tests.
The risk of bearing a child with a genetic defect such as Down syndrome is directly related to a woman's age (the older the woman, the greater the risk).At age 35, a woman's risk of carrying a fetus with Down syndrome roughly equals the risk of miscarriage caused by the procedure (about 1 in 200). In comparison, at age 25 the risk of this defect is about 1 in 1,400; by age 45 it increases to about 1 in 20. Nearly half of all pregnant women over 35 in the United States undergo amniocentesis and many younger women also decide to havethe procedure.
One of the most common reasons for performing amniocentesis is an abnormal alpha-fetoprotein (AFP) test (see below). Alpha-fetoprotein is a protein produced by the fetus and present in the mother's blood. A simple blood screening,usually conducted around the 15th week of pregnancy, can determine the AFP levels in the mother's blood. Levels that are too high or too low may signal aproblem with the fetus. Amniocentesis is recommended whenever the AFP levelsfall outside the normal range.
The procedure is generally performed during the 16th week of pregnancy; results are usually available within three weeks. While it is possible to performamniocentesis as early as the 11th week, there appears to be an increased risk of miscarriage when done at this time.
The advantage of an early test and speedy results lies in the extra time fordecision making if a problem is detected. Potential treatment of the fetus can begin earlier. Important, also, is the fact that elective abortions are safer and less controversial the earlier they are performed.
As an invasive surgical procedure, amniocentesis poses a real, although small, risk to the health of a fetus. Parents must weigh the potential value of the knowledge or reassurance against the small risk of damaging what is probably a normal fetus. The serious emotional and ethical dilemmas that adverse test results can bring must also be considered. The decision to undergo amniocentesis is always a matter of personal choice. It is important for a woman to fully understand the procedure and to feel confident in the obstetrician performing it. Evidence suggests that a physician's experience with the procedurereduces the chance of problems. Almost all obstetricians are experienced in performing amniocentesis. The patient should feel free to ask questions and seek emotional support before, during and after the amnio is performed.
The word amniocentesis literally means "puncture of the amnion," the thin-walled sac of fluid in which a developing fetus is suspended during pregnancy. During the sampling procedure, the obstetrician inserts a very fine needle through the woman's abdomen into the uterus and amniotic sac and withdraws aboutan ounce of amniotic fluid for testing. The relatively painless procedure isperformed on an outpatient basis, sometimes using local anesthesia.
The doctor uses ultrasound images to guide needle placement and collect the sample, lessening the risk of harming the fetus or having to insert the needleseveral times.
After the procedure, the woman should rest for the first 24 hours and avoid heavy lifting for two days.
The sample of amniotic fluid is sent to a laboratory where fetal cells contained in the fluid are isolated and grown in order to provide enough genetic material for testing. This takes about 7-14 days. The material is then examinedunder a microscope. For some disorders (like Tay-Sachs), the simple presenceof a telltale chemical compound in the amniotic fluid is enough to confirm adiagnosis. Depending on the specific tests ordered and the skill of the labconducting them, the results should be available between one and four weeks after the sample is taken.
Cost of the procedure depends on the doctor, the lab, and the tests ordered.Most insurers provide coverage for women over 35, as a follow-up to positivematernal blood screening results, and when genetic disorders run in the family.
An alternative to amniocentesis now widely used is chorionic villus sampling,or CVS, which can be performed as early as the eighth week of pregnancy. While this allows for the possibility of a first trimester abortion, if warranted, CVS is apparently also riskier and is more expensive.
The most promising area of new research in prenatal testing involves expanding the scope and accuracy of maternal blood screening, since this poses no risk to the fetus.
Once the procedure has been safely completed, the anxiety of waiting for thetest results can prove to be the worst part of the process. A woman should seek emotional support from family and friends, as well as from her obstetrician and family doctor. Professional counseling may also prove necessary, particularly if a fetal defect is discovered.
There are several risks involved with the procedure. While slight bleeding (called "spotting") in pregnancy is fairly common, bleeding following amniocentesis should always be checked out. Rarely, infection can occur after the procedure. eaking of amniotic fluid or unusual vaginal discharge together with fever could signal the onset of infection. An unchecked infection can lead to severe complications. Unusual abdominal pain and/or cramping may indicate theonset of Premature labor, but mild cramping for the first day or two following the procedure is normal. There is a very slight risk of injury to the fetusresulting from contact with the needle. The rate of miscarriage occurring during amniocentesis appears to be about 0.5%, compared to a miscarriage rate of 1% for CVS. Many fetuses with severe genetic defects miscarry naturally during the first trimester.
Negative results from an amniocentesis indicate that everything about the fetus appears normal and the pregnancy can continue without undue concern. A negative result for Down syndrome means that it is 99% certain that the baby does not have the condition.
An overall "normal" result does not guarantee that the pregnancy will come toterm, or that the fetus does not suffer from some other defect. Laboratory tests are not 100% accurate at detecting targeted conditions, nor is it possible to test for every fetal condition.
Positive results indicate the presence of a birth defect with an accuracy approaching 100%. Prospective parents are then faced with emotionally and ethically difficult choices regarding treatment options, the prospect of dealing with a severely affected newborn, and the option of elective abortion. At thispoint, the parents need expert medical advice and counseling.
The alpha fetoprotein (AFP) test is a measure of a substance called alpha fetoprotein in a pregnant woman's blood. Alpha fetoprotein (AFP) is produced byan unborn baby's liver and is found in both the amniotic fluid and the mother's blood. Abnormally high amounts of AFP in the mother's blood indicates thefetus may have one of several serious birth defects. This screening test cannot diagnose a specific condition; it only indicates a risk of several birth defects. The AFP test also can be used to detect liver disease, certain cancerous tumors, and to monitor the progress of cancer treatment.
The exact function of this protein is unknown. After birth, the infant's liver stops producing AFP, and an adult liver contains only trace amounts. Duringpregnancy, the fetus excretes AFP in urine and some of the protein crosses the fetal membranes to enter the mother's blood. The level of AFP can then bedetermined by analyzing a sample of the mother's blood. It is very importantthat the doctor know precisely how old the fetus is when the test is performed since the AFP level changes over the length of the pregnancy. Errors in determining the age of the fetus lead to errors when interpreting the test results. Since an AFP test is only a screening tool, more specific tests must follow to make an accurate diagnosis. An abnormal test result does not necessarily mean that the fetus has a birth defect. The test has a high rate of abnormal results (either high or low).
The AFP test is usually performed during the 16th week of pregnancy. AFP canbe measured in a blood test or in the sample of amniotic fluid taken at the time of amniocentesis. Test results are usually available after about one week.
The most common and severe type of birth defect that the AFP test can detectare spinal column defects such as (spina bifida) and anencephaly (abasence ofbrain tissue). During pregnancy, if the neural tube (the structure that willbecome the brain and spinal cord) doesn't close correctly, AFP may leak through this abnormal opening and enter the amniotic fluid. This leakage createsabnormally high levels of AFP in amniotic fluid and in the mother's blood.
Other fetal conditions that can raise AFP levels above normal include:
- Cysts at the end of the spine
- Blockage in the esophagus or intestines
- Liver disease
- Defects in the abdominal wall
- Kidney or urinary tract defects or disease
- Brittle bone disease.
- Too little fluid in the amniotic sac around the fetus
Levels may also be high if there is more than one developing fetus, or a pregnancy that is farther along than estimated.
For unknown reasons, abnormally low AFP may indicate that the fetus has an increased risk of Down syndrome. Down syndrome is a condition that includes mental retardation and a distinctive physical appearance. If the screening testindicates an abnormally low AFP, amniocentesis is used to diagnose the problem. Abnormally low levels of AFP can also occur when the fetus has died.
Although AFP in human blood gradually disappears after birth, it never disappears entirely. It may reappear in liver disease, or in tumors of the liver, ovaries, or testicles. The AFP test is used to screen people at high risk forthese conditions. After a cancerous tumor is removed, an AFP test can monitorthe progress of treatment. Continued high AFP levels suggest the cancer is growing.
The risks associated with drawing blood are minimal, but may include bleedingfrom the puncture site, feeling faint or lightheaded after the blood is drawn, or blood accumulating under the puncture site (hematoma).
If results are abnormal, the doctor will inform the woman of her specific increased risk as compared to the "normal" risk of a standard case. If the riskof Down syndrome is greater than the standard risk for women who are 35 yearsold or older (1 in 270), then amniocentesis is recommended. This screening test only predicts risk; appropriate diagnostic testing will follow after an abnormal screening result. In tumor or liver disease testing, an AFP level greater than 50 ng/mL is considered abnormal.