Amniocentesis

Amniocentesis is an invasive procedure used to obtain amniotic fluid for prenatal diagnosis of a fetus (e.g., assessment of fetal lung maturity).

In the 1950s the measurement of bilirubin concentrations present in amniotic fluid in monitoring the rhesus diseases was first reported. Amniocentesis for fetal chromosome analysis was also initiated in the 1950s. The first application was for fetal sex determination. Down syndrome via amniocentesis was first detected in 1968.

Cells naturally are exfoliated from the surface and from the mucosae of the fetus and some of these cells survive for a time in the fluid surrounding the fetus in the amniotic cavity. Soluble biochemical material of clinical A physician uses an ultrasound monitor (left) to position the needle for insertion into the amnion when performing amniocentesis. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.


significance produced by the fetus may also accumulate in the amniotic fluid. The fluid can be analyzed for these substances directly.

For amniocentesis, the maternal abdomen is washed with antiseptic solution. A local anesthetic is given and a hollow needle (22 gouge) is inserted through the mother's abdominal wall into the amniotic cavity avoiding the placenta if possible and a sample of the fluid (approximately 20 mL) is withdrawn with a syringe attached to the needle. In order to insure the safety of the fetus, the procedure is monitored in real time via an ultrasound scan. In twin pregnancies, after withdrawal of amniotic fluid of the first sac, an injection of a dye is necessary to understand if the fluid of the second sac has been drawn. If the fluid of the second puncture is clear, then it does not come from the first sac.

Viable cells in the fluid are then cultured (grown) in vitro. At 16 weeks' gestation amniotic fluid contains 200,000 cells mL, but a very small number are capable of forming colonies. The chromosomes of the cultured cells can then be examined.

Viewing the chromosomes under a light microscope will reveal if a normal diploid number of chromosomes are present or if extra or fewer chromosomes are present. Additionally, structural chromosomal aberrations, as well as uniparental disomies can be detected.

FISH (fluorescence in situ hybridization) analysis can also be used for rapid detecting numerical anomalies involving chromosome 13, 18, 21, X and Y. Quantitative polymerase chain reaction (Q-PCR) has been also used for detecting the most common aneuploidies. More recently, it has been shown how amniocentesis can be used for detecting DNA anomalies responsible for the etiology of many autosomal and X-linked disorders as well as hemophilia A, sickle cell anemia, DiGeorge syndrome, and other diseases.

Amniocentesis is an elective procedure that can detect the presence of many types of genetic disorders, thus allowing doctors and prospective parents to make important decisions about early treatment and intervention. Down syndrome is a chromosomal disorder characterized by a diversity of physical abnormalities, mental retardation, and shortened life expectancy. It is by far the most common, nonhereditary, genetic birth defect, afflicting about one in every 1,000 babies. Since the risk of bearing a child with a nonhereditary genetic defect such as Down syndrome is directly related to a woman's age, amniocentesis is recommended for women who will be older than 35 on their due date. Thirty-five is the recommended age to begin amniocentesis because that is the age at which the risk of carrying a fetus with such a defect roughly equals the risk of miscarriage caused by the procedure—about 1 in 200.

Maternal complications of amniocentesis such as septic shock and amnionitis are rare. Rhesus isoimmunization can be prevented by prophylactic administration of anti-D immunoglobulin to Rh-negative women. Risk of abortion has been quoted of 1% about for single pregnancies and 3% about for twin pregnancies. Amniocentesis is ordinarily performed between the 14th and 16th week of pregnancy, with results usually available within three weeks. It is possible to perform amniocentesis as early as the 11th week but this is not usually recommended because there appears to be an increased risk of miscarriage when done at this time. Furthermore, the CEMAT study (Canadian Early and Mid Trimester Amniocentesis Trail) in 1998 cleared that early amniocentesis from 11 to 12 weeks is associated with significant disadvantages because of difficult or unsuccessful procedures as well as more than one needle insertion and more likely fetal cells culture failure. The advantage of early amniocentesis is the extra time for decision making if a problem is detected. Potential treatment for the fetus can begin earlier. Elective abortions are safer and less controversial the earlier they are performed.