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Amniocentesis is the removal of amniotic fluid via a needle inserted through the maternal abdomen into the uterus and amniotic sac, in order to gain information about the fetus. Amniocentesis can be used for the following types of tests:
1. Chromosome analysis - the cells in the fluid are largely from the amniotic membrane (called "amniocytes") and some fetal skin cells. Since they are not actively growing, they need to be cultured and stimulated to grow which takes about 7-14 days. After that, the cells are "harvested" and treated to make the chromosomes visible in their expanded form. The cells are then broken to release the chromosomes which are stained with various techniques for visualization of the "bands" on each chromosome and to allow identification. The chromosomes are counted and matched up in 23 pairs, and then examined for evidence of missing pieces or extra pieces.
The most common reason for chromosome analysis is to look for evidence of trisomy - three copies of a chromosome instead of the expected two. The trisomies 13,18 (Edward's syndrome), and 21 (Down syndrome) are the most common, with trisomy 21 comprising about half of all the trisomies identified. Trisomy in the fetus is related to maternal age, ranging from a one in several thousand for women in their teens and 20's to more than one in twenty in fetuses of women older than 45. Traditionally, the test has been offered to women who would be 35 years of age or older at delivery, because the risk of a trisomy (1 in 180) is about equal to the risk of miscarriage from the procedure (about 1 in 200). This reasoning may not be appropriate given the nature of the perceived burden of miscarriage risk and trisomy risk for each individual couple, so genetic counseling by a trained and certified counselor is essential before making the decision. Most centers will offer amniocentesis on request to women younger than 35.
2. Alpha-fetoprotein (AFP) Follow-up Services - should only be done at a state-certified AFP follow-up center. These centers have strict requirements for genetic counseling and qualifications and experience of the health care provider performing the amniocentesis.
Amniocentesis for AFP follow-up is done for the following reasons:
- Screen positive Expanded AFP blood test for either Down syndrome, spina bifida, or Trisomy 18.
- Family history of spina bifida.
- If the mother is taking valproic acid (Depakote) or carbamazipine (Tegretol) for seizure disorder. These drugs give an increased risk of spina bifida.
The AFP level in the fluid is measured in all amnios, but in the case of AFP followup for "screen positive for spina bifida" or "screen positive for Down syndrome", the blood test is the main reason for having the procedure, not maternal age. If the amniotic fluid AFP is normal and the ultrasound shows a normal spine and abdominal wall, spina bifida or abdominal wall defects can be excluded with 99 percent certainty. Down syndrome is excluded with 99 percent certainty as well if the chromosomes are normal. Important: other birth defects and conditions cannot be excluded as they are not tested for unless a family history or ultrasound findings indicate a problem.
TECHNIQUES OF AMNIOCENTESIS
GENERAL INFORMATION
Amniocentesis is best performed by an experienced practitioner. The complication rate is directly related to the number of amnios done in the past, and particularly to the number done per year or per month. In addition, the following points are important:
- Genetic counseling is essential in aiding parents in making the decision about amnio. This should be a formal counseling session, not merely a brief discussion in your health care practitioner's office - the risks and benefits must be spelled out in detail.
- Ultrasound should be performed by an experienced Registered Diagnostic Medical Sonographer.
- The practitioner should also be experienced in second-trimester obstetrical ultrasound for the detection of fetal anomalies.
- The amniocentesis should be performed under continuous ultrasound guidance, in order that the needle can be visualized throughout the procedure. This greatly adds to the success rate of obtaining fluid, as well as preventing multiple needle insertions or repositionings, which may increase the chance of complications.
HOW THE AMNIOCENTESIS IS DONE
A detailed ultrasound should be performed first. The fetal measurements are obtained, consisting of the head diameter (BPD), the head circumference, the abdominal circumference, and the femur length. In addition, some practitioners may include humerus (upper arm bone) measurements. When combined mathematically, these measurements give an estimate of gestational (menstrual) age to within 2 weeks. Important - your due date will not change unless the measurements indicate a difference of at least 10 days. Less difference than this is essentially a match for your last menstrual period dates. If you had no reliable last menstrual period, the ultrasound due date will be used, but is not as accurate as a good last menstrual period.
Next, a detailed anatomic survey of the fetus should be done. This includes anatomy of the head, brain, spine, chest, heart structure, abdominal organs including stomach and gall bladder, kidneys, bladder, umbilical cord, and examination of the arms and legs for normal shape. The location of the placenta and the amount of amniotic fluid are noted. Any fibroids or maternal ovarian cysts can also be seen. The cervix should be looked at to note any dilation or proximity to the placenta.
Finally, an amniocentesis site is chosen. The practitioner will try to choose a site that is as high on the uterus as possible, is not over the placenta, and which is away from the fetus, especially the face. This is only a preliminary site, and may have to be changed just before the amniocentesis is done if the baby has moved in the interim. The site is marked with a soda-straw like tube, which makes a little indentation on the skin (ink cannot be used, as it washes off when the skin is cleaned off with the alcohol or iodine solution).
The area around the site is then cleaned with alcohol or iodophor (betadine) solution and the ultrasound put back on to look at the chosen path of the needle.
Preparation of the maternal anterior
abdominal wall for amniocentesis
If all is clear, the needle is then inserted, watching its path on the screen to guide it into the pocket of fluid.
Infiltration of local anesthetic
over the placement site
of the amniocentesis needle.
Some practitioners don't recommend the use of local anesthetic, as it causes more discomfort and limits the insertion to one exact point.
Insertion of the amniocentesis needle
into a pool of amniotic fluid
detected by ultrasound.
After the needle is in, the first 1/2 cc is discarded as it may contain the mother's skin cells and would result in analysis of the wrong chromosomes. Then 30cc (two cooking tablespoons) of fluid is withdrawn - this usually takes about 30 seconds or less.
Aspiration of amniotic fluid
with a 20 ml syringe.
The needle is removed and the baby again checked for a normal heartbeat to reassure the mother. The fluid normally is pale yellow and is placed into sterile tubes for chromosome analysis and into a smaller tube for AFP.
Aliquoting of the amniotic fluid
for cell culture and biochemistry.
The labels on the tubes are then confirmed by the parents to bear their name and address stamp. A band-aid is placed if needed, or if the mother has a toddler who needs to be shown mommy's "owie" or "boo-boo" site.
The mom is advised not to lift anything heavy for two days and to report any fluid leakage, fever, sever cramps, or bleeding to her health care provider or genetic clinic. Slight cramping, like gas pains, is normal in the first day or so and is no cause for alarm. A slight amount of fluid leakage from the vagina, enough to make a small spot on the clothing, is also normal but should be reported.
Is it uncomfortable? With an experienced practitioner, the vast majority of women report that all they felt was a slight pinching and perhaps some vague cramping. The dad should remain sitting throughout the procedure - for some reason, if anyone is going to become lightheaded, it's not usually the mother!
MoonDragon's Obgyn Information: Procedures Overview - Amniocentesis
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Photos from Diagnosis in color Obstetrics and Gynecology by Symonds and Macpherson.
