Amniocentesis is a way of sampling the amniotic fluid. It is done between weeks fifteen and eighteen. It is the test most frequently recommended to pregnant women who are thirty-five years old and over. Under ultrasound guidance, the doctor inserts a hollow needle into the amniotic sac, through the women’s belly (but not through the navel, as is commonly thought). Less than an ounce of fluid is taken out and sent to a laboratory for analysis. Because the baby sheds cells into the amniotic-fluid, these can be encourage to grow in the lab, and then be put under a microscope to see if any abnormalities are present. Early amniocentesis is significantly riskier than mid trimester amniocentesis and that amniocentesis at 15 to 18 weeks is the standard. If earlier genetic diagnosis is indicated, the chorionic villus sampling is an option at 10 to 12 weeks’ gestation.
By 15 weeks the amniotic fluid volume has increased to approximately 150 ml., making the procedure easier to perform. For each succeeding week of gestation, the volume increases substantially. Doing the procedure between 15 and 18 weeks allows enough time to complete cell cultures and to evaluate laboratory tests before it is too late to consider termination, should an abnormality be detected.
Dr. J. Queen in an article on genetic amniocentesis summarized the indications for the procedure and put it in a table form (see below.)
TABLE 1
Maternal age of 35 years or older at the time of delivery.
Parent with a balanced structural chromosomal rearrangement
Previous child with chromosomal abnormality
Previous child with a neural tube defect
Mother of carrier for X-linked recessive disorder diagnosable by amniotic fluid analysis
Elevated maternal serum alpha-fetoprotein
Low maternal serum alpha-fetoprotein
Abnormal triple screen
Diagnosis of cystic fibrosis, A or B thalassemia, Tay Sachs, hemophilia, and Duchenne-Becker muscular dystrophy. *
*A couple may be recognized to be at risk because of an affected child or relative. Couples at risk with Tay Sachs, cystic fibrosis, and hemoglobinopalthis are more likely to be identified through screening programs to identify heterzygotes.
Normally, it takes about two weeks for the results to be available, although newer techniques are being developed that allow for rapid readouts.
Besides doing a routine check for certain chromosomal abnormalities, amniocentesis may also be recommended at other times. When the woman has an ultrasound between weeks eighteen and twenty, certain findings may suggest that the baby has abnormal chromosomes. These sonographic findings include swollen kidneys, a density in the bowel (known as an echogenic bowel), a specific type of cyst in the brain (choroids plexus cyst), and an umbilical cord with only two blood vessels instead of the usual three. An echogenic bowel may also show on ultrasound when the baby has cystic fibrosis or cytomegalovirus.
A choroids plexus cyst is a small, harmless, transient cyst in part of the brain. These cysts have been linked with chromosomal anomalies in 1 percent of patients, typically fetuses with trisomy 18.
Amniocentesis for Twins
In 1992 a group of researchers from Yale introduced a new technique for genetic amniocentesis in twins.
Using an ultrasound probe, the separating membrane and an adjacent pocket of amniotic fluid in each cavity are simultaneously visualized. An amniocentesis needle is advanced into the first cavity. A second needle is introduced into the other amniotic cavity without altering the position of the transducer. This technique permits visualization of both needles simultaneously, providing proof of proper placement of the needle in each cavity.