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Amniocentesis for prenatal diagnostic testing is usually offered between 15 and 20 weeks of gestation. Under ultrasound guidance, a 20-22-gauge spinal needle is passed into the amniotic fluid. The initial aspirate of 1-2 mL of fluid should be discarded to decrease the chance of maternal cell contamination. A total of 20-30 mL of fluid is aspirated, and the needle is amniocentesis.

Amniocentesis has greater than 99% cytogenetic diagnostic accuracy and a fetal loss rate of approximately 0.5%. The most common complications have been transient vaginal spotting or amniotic fluid leakage in approximately 12% of all cases. The incidence of chorioamnionitis is less than 1 in 1,000. Needle injuries to the fetus.

Amniocentesis with twins is possible in 95% of pregnancies. Amniotic fluid is aspirated from the first sac. Before the needle is removed, 2-3 mL of indigo carmine (methylene blue is not recommended) diluted 1:10 in bacteriostatic water is injected into the first sac. Amniocentesis is then performed on the second sac at a location.

Ninety percent of NTDs are associated with intracranial abnormalities or varying degrees of hydrocephaly. The value of amniotic fluid alpha-fetoprotein determination with an ultrasound evaluation has become increasingly controversial as ultrasound resolution has improved. Through amniotic fluid alpha-fetoprotein analysis, diagnosis of an NTD is possible in all except the 5% of fetuses whose spinal defect is covered by skin. The amniotic fluid alpha-fetoprotein level may be spuriously elevated if the amniotic fluid is contaminated.

This potential error can be eliminated, however, if amniotic fluid acetylcholinesterase is assayed simultaneously. Its presence verifies that the elevated amniotic fluid alpha-fetoprotein level is likely due to an NTD or other fetal defect. If fetal hemoglobin is present but acetylcholinesterase is absent, the elevated amniotic fluid alpha-fetoprotein level can be deduced to result from either the presence of fetal blood or from an anomaly other than an NTD. Failure to detect an anomaly by ultrasonography does not necessarily indicate that the elevated maternal serum alpha-fetoprotein level.

A subtle anomaly may still exist. Alternatively, an unexplained elevated maternal serum alpha-fetoprotein level may reflect placental dysfunction and indicate increased risk for certain pregnancy complications (eg, preterm labor, IUGR, stillbirth).

Chorionic Villus Sampling

Indications for CVS are essentially the same as those for amniocentesis, except for analyses that require amniotic fluid rather than amniotic fluid cells. The primary advantage of CVS is that results are available much earlier in pregnancy, which decreases parental anxiety when results are normal and, when they are abnormal, allows earlier and safer methods of pregnancy termination. Earlier diagnosis may also be required for prenatal treatment (eg, prevention of female virilization in fetuses affected with 21-hydroxy-lase deficiency by administration of dexamethasone to the mother). Chorionic villus sampling is generally performed.

Placental villi may be obtained through transcervical, transabdominal, and transvaginal access to the placenta. Active infection is a contraindication to transcervical CVS. Later in pregnancy, when a fetal malformation or IUGR is associated with severe oligohydramnios, transabdominal CVS is an alternative method of

There have been three major collaborative studies comparing CVS and amniocentesis. Trials at the National Institute of Child Health and Human Development and in Canada had similar results, with more than 99% cytogenetic analysis success and total pregnancy loss rates of 0.6-0.8% in excess of amniocentesis. A European study reported an excess loss rate in the CVS group that was different both clinically and statistically from the results of the other two large

Prenatal cytogenetic diagnosis is occasionally complicated by the finding of more than one distinct cell line obtained from a single chorionic villus sample. When this occurs, it may be the result of fetal karyotypic mosaicism, confined placental mosaicism, or pseudomosaicism. Fetal mosaicism represents two distinct cytogenetic cell lines in the fetus. Confined placental mosaicism is defined as two cell lines that occur only in the placenta and not in the fetus. Pseudomosaicism represents two or more cell lines that have arisen in cell culture but are not representative of the fetus or placenta. Most cases of true mosaicism are actually confined to the placenta. However, all mosaic findings should be analyzed by other methods of invasive prenatal diagnostic testing at a later point in pregnancy (eg, amniocentesis or PUBS). Some cytogenetic laboratories perform both direct and cultured cell karyotype analyses on villus samples. Cytotrophoblasts may be stained and analyzed directly within 24-48 hours. This method is associated

Poor pregnancy outcome and perinatal loss have been reported in association with confined placental mosaicism. The utility and method of antepartum testing in such cases have

There have been reports of an association between CVS and limb reduction and oromandibular defects. Cavernous hemangiomas have 

The risk for these anomalies is unclear. According to the World Health Organization, the global 10-year experience with CVS reveals an incidence of limb reduction defects of

A National Institute of Child Health and Human Development workshop on CVS and limb reduction defects concluded that the frequency of oromandibular-limb hypogenesis appeared to be more common after CVS, especially when CVS is performed before 9 menstrual weeks. Women considering CVS should be informed of the present concerns about the possible association of CVS with limb and