Techniques of amniocentesis

General

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 mathematicaFlly, 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.

Finally, an amniocentesis site is chosen. We 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 amnio 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. If all is clear, the needle is then inserted, watching its path on the screen to guide it into the pocket of fluid. We don't recommend the use of local anesthetic, as it causes more discomfort and limits the insertion to one exact point. 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 Then 30cc (two cooking tablespoons) of fluid is withdrawn - this usually takes about 30 seconds or less. 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. 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" 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 doctor or to us. 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!

Indications for amniocentesis

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 trisomy 13 (Patau Syndrome),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. AFP follow-up services - in California, 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 physician performing the amnio. In addition, if done at a Center, all follow-up services (counseling, ultrasound, amnio, and lab fees) are free of charge.

Amnio for AFP follow-up is done for the following reasons:

The AFP level in the fluid is measured in all amnios, but in the case of AFP follow-up 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% certainty. Down syndrome is excluded with 99% 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.* 

Chorionic villus sampling

CVS is done at 10-11 weeks, and is done either transabdominally or transcervically depending on where the placenta is located (if it's on the front, a transabdominal approach can be used). It involves inserting a needle (abdominally) or a catheter (cervically) into the substance of the placenta but staying outside the amniotic sac. Then suction is applied with a syringe and about 10-15 milligrams of tissue are aspirated into the syringe. This tissue is manually cleaned of maternal uterine tissue and then grown in culture, and a karyotype is made like with amniocentesis.

The advantage: quicker results - about 7-10 days after the procedure, so if you choose to terminate the pregnancy this decision could be made at 11-13 weeks rather than at 17-18 as with amnio. In both situations a D&C is used, but some couples prefer to not announce the pregnancy if they are very concerned about a problem until after they get good results.

Disadvantages:

It's a good choice for those at high risk or those who want results sooner.