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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 removed.

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

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

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 with fetal blood.


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

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

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% amniocentesis, chorionic villus sampling.


Percutaneous umbilical cord blood sampling is also known as cordocentesis. During cordocentesis, the umbilical vein is punctured and blood is withdrawn under ultrasound guidance. Cordocentesis has also been used to obtain fetal blood cells for prenatal diagnosis when a rapid diagnosis is important. For example, the PUBS technique can be performed when a fetal malformation or severe IUGR is detected late in pregnancy. A procedure-related pregnancy loss rate of 1.4% has been reported.


Some conditions not diagnosable by conventional means of invasive prenatal diagnostic testing may be detected by fetal tissue sampling. Fetal tissue sampling can be performed either by fetoscopy, with its rather high (1-3%) procedure-related pregnancy loss rate, or by ultrasound-guided biopsy. Other procedures include fetal skin sampling for the diagnosis of epidermolysis bullosa and congenital ichthyosis and fetal liver biopsy to detect certain hepatic enzyme deficiencies.