Amniotic Fluid Amniocentesis

Amniocentesis is performed by a physician for prenatal diagnosis of congenital disorders, to assess fetal maturity, or to look for Rh isoimmunization or intrauterine infection. Although ultrasound is not essential, amniocentesis is best performed with its assistance to aid locaHzation of the placenta and to determine the presentation of the fetus. The best sites for obtaining amniotic fluid are behind the neck of the fetus, below its head, or from other unoccupied areas of the amniotic cavity. 

The skin is cleaned and anesthetized as for other similar procedures, and lOmL of fluid is aspirated into a syringe connected to the spinal needle that is typically used. Sterile containers such as polypropylene test tubes or urine cups are used to transport the fluid to the laboratory. If a specimen is for the determination of the lecithin-sphingomyelin (L/S) ratio, the container is immediately placed in ice if it is for spectrophotometric analysis, the specimen should be transferred to a brown tube or bottle to prevent photodegradation of bilirubin. Alternatively the specimen container may be wrapped in aluminum foil. 

There are few complications from amniocentesis. Occasionally a bloody tap is made. The blood may come from the uterine wall, the placenta, or even the fetus. Determination of fetal hemoglobin can be used to help ascertain the source if it is important to do so. 

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A small sample of amniotic fluid (10-20 ml) is collected at approximately 16 weeks gestation. Fetal cells are present in the amniotic fluid and can be used to diagnose single-gene disorders, chromosome abnormalities, and some biochemical disorders. Elevated a-fetoprotein levels indicate a fetus with a neural tube defect. The risk of fetal demise because of amniocentesis is estimated to be approximately 1/200. 

Biotinidase activity can be measured in cultured amniotic fluid cells and in amniotic fluid. Therefore, prenatal diagnosis of biotinidase deficiency is possible. Prenatal diagnosis has been performed in two at-risk pregnancies in which amniocentesis was performed because of advanced maternal age. The fetuses were found to be unaffected, and this was confirmed after birth. In addition to enzyme determination in amniocytes, a fetus was correctly shown to be a heterozygote by molecular mutation analysis in an at-risk pregnancy. Because treatment is so effective in this disorder, some laboratories are now performing prenatal diagnosis. 

Holocarboxylase synthetase deficiency can be diagnosed prenatally by assessing the response of carboxylase activity in cultured amniocytes (obtained by amniocentesis) to the addition of biotin, or by the detection of methylcitric and hydroxyisovaleric acids in the amniotic fluid. Prenatal therapy, by giving the mother 10 mg of biotin per day, results in sufficiently elevated fetal blood concentrations of biotin to prevent the development of organic acidemia at birth. 

Tay-Sachs disease can be diagnosed in the course of fetal development. Amniotic fluid is obtained by amniocentesis and assayed for P-A-acetylhexosaminidase activity. 

Many different samples are available for clinical laboratory analysis before and during pregnancy. These include paternal serum and blood maternal serum, blood, and urine amniotic fluid obtained by amniocentesis or from pools of fluid in the vagina after rupture of the fetal membranes chorionic villi fetal blood obtained by percutaneous umbilical blood sampling and fetal tissue obtained by biopsy. ... 

For sensitized mothers with an at-risk fetus, serial titers are performed on maternal serum every month until 24 weeks gestation, then every 2 weeks thereafter. If a critical titer anti-D is detected, then ultrasound Doppler measurements are used to determine the peak velocity of blood flow in the fetal middle cerebral artery. Higher velocity is a strong indicator of fetal anemia. In addition, amniocentesis is performed to assess the bilirubin concentration in amniotic fluid. 

Amniotic fluid is obtained by transabdominal amniocentesis usually during real-time sonographic visualization. The clinician should try to avoid traversing the placenta, but wfll sometimes fail. In a multifetal pregnancy, there are usually separate sacs. Each individual sac should be sampled. Fluid... 

Dombroski RA, Mackenna J, Brame RG. Comparison of amniotic fluid lung maturity profiles in paired vaginal and amniocentesis specimens. Am J Obstet Gynecol 1981 140 461-4. 

Where no effective treatment exists at present for some inborn errors of metabolism, and where these disorders are associated with severe illness, mental retardation, or early death, the prenatal diagnosis of the diseased fetus by amniotic fluid enzyme analysis is now possible (MIO, Mil, M12). These new developments in the field of transabdominal amniocentesis, coupled with more liberal abortion laws, can serve to reassure families with a previous history of a fatal disease that their offspring will be physiologically normal. As applied to the field of amino acid disorders, enzyme analysis of cultured amniotic fluid cells has been used to diagnosis potential cases of homocystinuria (Fig. 51) and... 

This woman should have an amniocentesis performed with bilirubin being measured in her amniotic fluid. In Rhesus incompatibility the amniotic fluid bilirubin concentration will rise because of the destruction of fetal red cells. 

Prenatal diagnosis of genetic disease is normally carried out on a sample of amniotic fluid obtained by amniocentesis or by chorionic villus sampling (CVS). Conventional diagnostic tests for diseases such as cystic fibrosis can take as long as 2 weeks. Results of a PCR test can be available in 1 day. If the option to terminate a pregnancy is taken, this time savings can... 

The amniotic fluid, which is elaborated by the amniotic membrane, contains only cells derived from fetus and amniotic membrane and no maternal cells. Thus, the sex of a male fetus has always been determined correctly by studying cells collected by amniocentesis. If amniotic fluid were significantly contaminated with maternal cells, this obviously would not have been possible. 

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