by Karen Schroeder, MS, RD, MEd
Anatomy and Physiology
When a sperm fertilizes an egg, an embryo is created. Fertilization usually takes place in the fallopian tube. Once the fertilized egg reaches the uterus, it attaches itself to the uterine lining, or endometrium.
At this point, a placenta forms between the uterus and developing embryo to provide the embryo with oxygen and nutrition from the mother's blood. An amniotic sac forms around the embryo to provide protection. After the first eight weeks of development, the embryo is called a fetus.
The amniotic sac continues to expand around the fetus as it grows and develops. The amniotic sac is filled with a clear liquid called amniotic fluid. This fluid cushions the fetus, controls body temperature, prevents the growth of bacteria, and supports fetal development.
The amniotic fluid contains cells that are shed from the fetus. This makes the fluid quite valuable for diagnosing some conditions. In an amniocentesis, a sample of this fluid is removed and tested for specific genetic disorders and birth defects.
Amniocentesis is usually done in the second trimester of pregnancy, at 15-18 weeks. It is generally offered to women with certain risk factors: age 35 and older, which increases the risk of chromosomal disorders, such as Down syndrome, a previous child or pregnancy with a birth defect, blood test or ultrasound suggesting a birth defect, such Down syndrome or neural tube defects, a family history of genetic disorders, such as cystic fibrosis.
Amniocentesis may also be done in the third trimester for these reasons: to determine if the baby's lungs are mature enough should early delivery become necessary; to diagnose a uterine infection; to check for anemia in a baby with Rh incompatibility.
Amniocentesis allows doctors to detect - and more often, rule out - problems with a baby's health long before birth. This information can help parents plan for proper care for their infant. The results of such testing can also aid parents in deciding whether or not to continue a pregnancy.
Another method of prenatal diagnosis is chorionic villus sampling, or CVS. In this procedure, a sample of the chorionic villi, which form part of the placenta, is removed and tested. The advantage of CVS is that it can be performed earlier in the pregnancy, at 10-12 weeks. The disadvantages are that it cannot detect neural tube defects, and it may carry a slightly higher risk of miscarriage compared to amniocentesis.
A less invasive screening test is the triple screen. At 16-18 weeks of pregnancy, blood taken from a woman's arm is tested for levels of three specific hormones. Abnormal results can indicate a higher risk of neural tube defects and genetic abnormalities. These women will be offered further testing with ultrasound or amniocentesis. A disadvantage of this test is that many abnormal results are false alarms.
Ultrasound is another non-invasive test. It is often done at 18-20 weeks of pregnancy. A gel is spread on a woman's abdomen and a transducer is placed on this gel and moved around. The transducer releases sound waves that bounce off the fetus and are converted into images that appear on a video screen. The doctor views these images to examine the fetal organs. Any possible abnormalities may lead to further testing.
Since prenatal testing can reveal information that may be difficult to deal with, it is essential that couples talk at length with highly trained genetic counselors for their advice on obtaining the tests and interpreting the results.
Amniocentesis is done in your doctor's office. There are no special preparations. However, the test should be done while you have a full bladder, so your doctor may ask you to avoid urinating for some time before the procedure.
You will lie on your back, and your lower abdomen will be cleaned. Your doctor will use ultrasound to create an image of the fetus and nearby structures on a computer screen. Looking at this image, the doctor can choose a safe place to insert the needle.
Your doctor may give you an injection of local anesthesia to numb the surface of your abdomen where the amniocentesis needle is to be inserted. Carefully watching the ultrasound monitor to avoid contact with the placenta or baby, your doctor will introduce the thin needle through your abdominal wall, into your uterus, and into the amniotic sac.
He or she will then withdraw a small sample of amniotic fluid, remove the needle, and cover the site with a bandage. Your body will make additional fluid to replace what was removed.
Your doctor will continue to monitor the ultrasound to check that the fetal heartbeat is normal and the baby tolerated the procedure well. The entire procedure takes about 20-30 minutes. You may feel mild cramping or pressure in your lower abdomen.
The fetal cells from the amniotic fluid will be grown in a laboratory for 1-2 weeks and then tested for the presence of genetic abnormalities. You should have the results in 2-4 weeks.
Risks and Benefits
Amniocentesis is beneficial for women at increased risk of having a baby with birth defects or genetic abnormalities. When these conditions are detected early, parents can make decisions about the pregnancy and about treatment.
While amniocentesis is fairly routine and safe, there are some risks: miscarriage-the average risk is 1 in 200; with amniocentesis before 15 weeks, this risk rises to 1 in 100; bleeding, cramping, and leaking fluid from the vagina--these symptoms occur in 1% of women who have an amniocentesis.
Other less common risks of amniocentesis include: infection, although this is very rare and steps are taken to prevent it; harm to the fetus from the needle, the risk of which is significantly reduced by the use of ultrasound; Rh factor problems, which can be prevented with an injection of Rh immune globulin if this is a concern; sampling of insufficient fluid, requiring a repeat procedure.
Compared to non-invasive screening tests such as maternal blood samples and ultrasound exams, amniocentesis is more accurate but not as safe for the fetus.
In amniocentesis, or any procedure, you and your doctor must carefully weigh the risks against the benefits to determine whether it's the most appropriate procedure for you.
After the Procedure
When the procedure is finished, you will be asked to rest on the table for 15-20 minutes. Then, you will be free to leave. For the next 24 hours, you will be advised to avoid physical stress such as vigorous exercise, lifting, prolonged standing, and sexual activity.
Once home, be sure to call your doctor if you experience any of these symptoms: signs of infection, including fever and chills, nausea or vomiting, increasing or persistent pain or cramping in the lower abdomen or shoulder, vaginal bleeding or loss of fluid from the vagina, redness, swelling, bleeding, or discharge from the amniocentesis site, new, unexplained symptoms.
- American Academy of Family Physicians. Maternal serum triple analyte screening in pregnancy. Available at: http://www.aafp.org/afp/20020301/915.html Accessed October 6, 2004.
- American Academy of Family Physicians. Prenatal diagnosis: amniocentesis and CVS. Available at: http://familydoctor.org/x1756.xml Accessed September 20, 2004.
- American Academy of Family Physicians. Triple screening in pregnancy: what it is and what to expect. Available at: http://familydoctor.org/x2426.xml Accessed October 6, 2004.
- March of Dimes. Amniocentesis. Available at: http://www.marchofdimes.com/professionals/681_1164.asp Accessed September 20, 2004.
- March of Dimes. Triple screen. Available at: http://www.marchofdimes.com/pnhec/159_522.asp Accessed October 6, 2004.
- Mayo Foundation for Medical Education and Research. Prenatal testing: what's involved and who should consider it? Available at: http://www.mayoclinic.com/invoke.cfm?id=PR00014 Accessed October 6, 2004.
- The National Women' Health Information Center. Healthy pregnancy. Available at: http://www.4woman.gov/Pregnancy/ Accessed September 20, 2004.
- U.S. National Library of Medicine. Amniocentesis. Available at: http://www.nlm.nih.gov/medlineplus/ency/article/003921.htm Accessed September 20, 2004.
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