What is Amniocentesis?
Amniocentesis is an office procedure available to aid in diagnosing fetal abnormalities. During pregnancy, the baby is enclosed in a delicate membrane sac called the amnion. Inside the sac is the amniotic fluid which cushions the growing embryo, maintains a constant temperature, and provides a medium in which the baby is free to move about. During development, cells and metabolic waste products are shed into the amniotic fluid, providing valuable clues as to sex, physical condition, and genetic make-up. The procedure known as amniocentesis entails removing some the amniotic fluid for laboratory analysis. While the analysis is complex and may take several weeks, collection of the fluid itself takes just a few minutes, causing little, if any, discomfort.
Using sonography, we determine the exact position of the fetus and the placenta in the uterus. The abdomen is prepped with an antiseptic. A needle is inserted into the amnion, under ultrasound guidance, and 15 to 20 cc of amniotic fluid is withdrawn. The fluid is transferred to the genetics laboratory for chromosomal analysis.
What Kinds of Abnormalities Can Be Detected By Amniocentesis?
Amniocentesis provides important information concerning a wide range of problems that might affect the developing fetus. For unknown reasons, there does appear to be an increased risk of Down's syndrome with advancing maternal age. The risk of chromosomal problems increases in pregnant women who are over the age of 35. Under 35 years of age, the risk of chromosomal abnormality occurring is 1 in 600, between 35 and 39 years of age, the risk increases to 1 in 60, between 40 and 44 years of age, the risk further increases to 1 in 40. In pregnant women over 45 years of age, the risk is 1 in 20.
Amniocentesis also affords an opportunity to provide sample fluid that can be assayed for the presence of a particular substance called alpha fetoprotein. A high level of alpha fetoprotein indicated a high probability for certain defects of nervous system development, namely open large defects of the neural tube. These defects include: open meningomyelocele, spina bifida, and/or anencephaly. Alpha fetoprotein is normally present in all pregnancies. In the pregnancies in which the fetus has anencephaly or an open spina bifida, the level of alpha fetoprotein rises above normal and continues to rise markedly.
Certain rare biochemical problems can also be diagnosed in utero, but these are only performed when the family history is pertinent. The procedure is also useful in the presence of Rh incompatibility, blood disorders, or where sex linked disorders such as hemophilia or muscular dystrophy are suspected.
Every patient considering a genetic amniocentesis has the option of a consultation with a geneticist. A genetics consult can help clarify whether the benefits of an amniocentesis outweigh the risks. To arrange a genetics consultation, speak with one of our doctors.
Risks Associated with Amniocentesis
Hundreds of thousands of genetic amniocenteses have been performed since the technology was developed more than two decades ago. Although amniocentesis does have some risk, depending a great deal on the experience of the person performing the procedure, it is now considered a routine procedure. In fact, the American College of Obstetrics and Gynecologists recommends that all women over the age of 35 at delivery be offered genetic amniocentesis.
This is the most serious risk associated with amniocentesis. It occurs in about 1 in 200 (0.5%) cases. However, miscarriages occur in the second trimester even without amniocentesis.
Loss of Fluid, Bleeding, Cramping
A small percentage of women undergoing amniocentesis experience one of these symptoms, but usually they do not lead to miscarriage. If you have any of these symptoms after amniocentesis, call the office for advice about which activities you should or should not do.
If the pregnant woman has an Rh negative blood type and the baby's father is Rh positive, the woman should have an injection of Rh immune
globulin after her amniocentesis to prevent Rh disease in the baby.
Although uncommon, it is important to call the office if fever (temperature greater than 100o) or flu-like symptoms arise within two weeks of your procedure.
Does Amniocentesis Always Work?
On occasion (less than 1%), the cells in the amniotic fluid drawn for analysis fail to multiply sufficiently to allow an accurate diagnosis to be made. As a result, even when the sample has been collected properly, test results cannot be obtained. In addition, laboratory errors can occur, although these are extremely rare.
It is important to remember too, that amniocentesis is intended to identify specific defects or abnormalities. Absence of these complications, of course, does not guarantee freedom from some other unsuspected defects or abnormalities.
A normal baby cannot be guaranteed when amniocentesis is performed. There are many conditions which cannot be diagnosed with amniocentesis, such as congenital heart diseases, extra digits, cerebral palsy, cleft lip and palate and mental retardation.
You can continue to perform most normal activities following your amniocentesis. However, for about three days, you should refrain from strenuous exercise (i.e. running), avoid vaginal intercourse and orgasms, and avoid heavy lifting (greater than 30-40 pounds).