PART 1: CHORIONIC VILLUS SAMPLING
Chorionic villus sampling (CVS): an invasive procedure performed for first-trimester prenatal diagnosis. CVS is typically performed between 70 and 91 days after the LMP. In the procedure, tissue is withdrawn from the villi (vascular fingers) of the chorion, a part of the placenta, and examined.
Chorion frondosum: the cellular, outermost extraembryonic membrane composed of trophoblastic cells that develops villi and forms the fetal component of the placenta.
Fluorescent in situ hybridization (FISH): rapid method of assessing targeted chromosomal abnormalities such as trisomy 21, 18, or 13.
First-trimester screening: a method of screening for chromosomal abnormalities in the first trimester using two biochemical tests, PAPP-A and hCG, as well as sonographic nuchal translucency measurement.
Sonographically guided chorionic villus sampling (CVS) has been available in the United States since the early 1980s and has offered couples at genetic risk an early and rapid prenatal diagnosis.1 The procedure, which can be performed as early as 10 weeks of menstrual age, provides preliminary cytogenetic results within 48 hours and final culture results within 7 days. In contrast, genetic amniocentesis is not routinely performed until approximately 16 weeks of menstrual age with an additional 7 to 10 days required to culture the amniotic fluid cells. Fluorescent in situ hybridization (FISH) can be used for both techniques. Thus, the pregnancy is nearly half completed before a definitive diagnosis can be established with amniocentesis. If a significant fetal abnormality is identified, the prospective parents must make a difficult choice of whether to continue or terminate the pregnancy. Postponing this decision until the midtrimester is extremely difficult because fetal movement has been perceived and significant bonding between the parent and fetus has occurred. In addition, the pregnancy is public knowledge, thereby precluding an element of privacy in decision making. If termination is chosen, maternal risks are greater than in the first trimester, with maternal mortality being up to 5 times higher.2
Despite the advantages of CVS, the procedure has struggled to become universally accepted. This has been due predominantly to a perception that the sampling and laboratory procedures are more complex than amniocentesis. In addition, there have been concerns that the procedure may induce fetal limb defects. Recently, however, enthusiasm for CVS has been renewed. First, contemporaneous studies have demonstrated the accuracy of the laboratory results, the reliability of the sampling, and the safety of the procedure if performed after 10 weeks of gestation by experienced operators. Second, studies have established the superior safety to CVS over first trimester amniocentesis.3-5 Additionally, over the last decade, the complication rates of CVS and midtrimester amniocentesis are comparable due to the reduction of CVS problems.6 Third, the recent success of first trimester screening for fetal chromosomal abnormalities provides an impetus for a first trimester diagnostic procedure for fetal karyotype.7
CONCEPTS AND INDICATIONS FOR CHORIONIC VILLUS SAMPLING