Chorionic villus sampling
Posted by Canadian health care on April 28th, 2009
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-hydroxylase deficiency by administration of dexamethasone to the mother). Chorionic villus sampling is generally performed at 9-12 weeks of gestation. Before CVS is performed, fetal viability and gestational age should be confirmed by ultrasonography.
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 invasive sampling.
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. There is probably a slightly higher risk of pregnancy loss from CVS than from amniocentesis, especially with the transcervical approach.
PERCUTANEOUS UMBILICAL CORD BLOOD 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 During cordocentesis, the umbilical vein is punctured and blood is withdrawn under ultrasound guidance.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.
FETOSCOPIC TISSUE SAMPLING
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.