Vol. 19: Spring 2001
Prenatal Diagnosis
Invasive Prenatal Diagnostic Techniques
Amniocentesis
Amniocentesis was first used for prenatal diagnosis in the 1950's. The first case of prenatal diagnosis of trisomy 21 was reported in 1968. Since then the role of amniocentesis has greatly expanded to include the diagnosis of chromosome abnormalities, biochemical disorders and a number of single gene disorders. In the US, more than 15,000 amniocentesis procedures are performed annually in the second trimester. The desire for earlier prenatal diagnosis has led to an increase in the numbers of both early amniocentesis and CVS procedures performed.
Standard amniocentesis is routinely performed at 15 to 16 weeks gestation. Early amniocentesis is done between 10 and 14 weeks. First, an ultrasound examination is performed to assess fetal viability and number, gestational age, fetal anatomy, and placental location. Next, an optimal pocket of amniotic fluid is identified, ideally avoiding the fetus, placenta and umbilical cord. Approximately 20 milliliters of fluid are collected with second trimester amniocentesis and approximately 1 milliliter of fluid per week gestation is removed with early amniocentesis. When performing amniocentesis on multiple gestation pregnancies, it is imperative to properly identify the fetuses. One should attempt to describe the location of the fetuses at the time of the procedure by tracing the umbilical cords to their placentas. Attempts should be made to describe any other ultrasound features that may help to identify the fetuses. If the results of the amniocentesis are abnormal, it will be necessary to correctly identify the fetus with the abnormal results.
Complications/risks of amniocentesis
Several large prospective trials have been undertaken to establish the safety of second trimester amniocentesis. All of these studies were nonrandomized and performed in the 1970's before ultrasound guidance was routinely used. Although the exact risk associated with amniocentesis is controversial, it is not a completely innocuous procedure and can result in a spontaneous abortion. Factors that have been reported to be associated with increased rates of fetal loss include a large number of needle insertions, using a needle greater than 18 gauge, and discolored amniotic fluid. Although earlier data suggested that placental perforation was associated with fetal loss, more recent data have not confirmed this.
Other reported procedure-related complications include leakage of amniotic fluid, vaginal bleeding, amnionitis, and needle puncture of the fetus. Leakage of amniotic fluid occurs in approximately 1-2% of women after undergoing amniocentesis. In most cases the fluid loss is minimal and resolves within several days. Vaginal bleeding may occur in 2-3% of cases and is self-limiting in most cases. Intra-amniotic infection following amniocentesis occurs in approximately 0.1% of cases. With the use of continuous ultrasound guidance, needle puncture of the fetus is usually avoidable.
Amniocentesis can potentially result in future reproductive complications secondary to sensitization to Rh and other antigens. Rh immune globulin prophylaxis should be administered to Rh-negative patients. However, this will not protect women from being sensitized to more rare antigens such as Kell.
Early amniocentesis
Early amniocentesis, performed before 14 to 15 weeks gestation, potentially provides fetal chromosome results much earlier than second trimester amniocentesis. Studies have shown that the chromosome study results from early amniocentesis are as accurate as those obtained in the second trimester. In contrast to CVS, potential advantages associated with early amniocentesis include the use of a familiar technique that is widely available, the ability to assess amniotic fluid alpha-fetoprotein measurement, and reduction of maternal cell contamination. Lastly, the likelihood that an abnormal chromosome result is due to chromosomal mosaicism confined to the placenta is reduced. This is because the chorionic villi, while of fetal origin, can contain abnormal chromosomes found only in the placenta and not in the fetus. The technique for early amniocentesis is similar to the technique used for second trimester amniocentesis. Early amniocentesis may be technically more difficult to perform because the amnion and chorion, which must be pierced by the needle, are often still separated by the extraembryonic coelom until 14 weeks gestation. This may result in tenting and stretching of the amniotic membranes and prevent access to the amniotic cavity.
Another difficulty encountered in early amniocentesis may be that the placenta is so extensive that access to the optimal pocket of fluid may require a transplacental approach. At this time, there has not been an observed increased rate of complications associated with transplacental early amniocentesis.
A number of studies have been conducted to evaluate the safety of early amniocentesis. An increased rate of post-procedure amniotic fluid leakage has been reported, which was associated with an increased incidence of talipes equinovarus. Based on the information available, it appears that early amniocentesis at eleven weeks through 12 weeks, 6 days is associated with an increased risk of fetal loss and talipes equinovarus. Thus, amniocentesis is not usually performed before 13 weeks gestation unless there are special circumstances. There is currently not enough data available to reach conclusions regarding the safety of early amniocentesis between 13 weeks and 14 weeks, 6 days gestation.
The Genetic Drift Newsletter is not copyrighted. Readers are free to duplicate all or parts of its contents. The Genetic Drift Newsletter is published semiannually by the Mountain States Regional Genetic Services Network for associates & those interested in Human Genetics. In accordance with accepted publication standards, we request acknowledgement in print of any article reproduced in another publication. The views expressed in the newsletter do not necessarily reflect local, state, or federal policy. For additional information, contact Carol Clericuzio, M.D., Editor, Department of Pediatrics, The University of New Mexico, Albuquerque, NM, 87131
Prenatal Diagnosis - Table of Contents
Introduction
Prenatal Diagnosis of Single Gene Disorders
Down Syndrome Maternal Serum Screening
Fetal Ultrasound
Amniocentesis
Chorionic Villus Sampling
Percutaneous Umbilical Blood Sampling
Preimplantation Genetic Diagnosis
The Future of Prenatal Diagnosis
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