CVS involves sampling of placental tissue rather than amniotic fluid and can be performed between 11 and 14 weeks. There are two routes used for CVS either transabdominal which is now the preferred option or the transcervical route if the former is not possible.
The transabdominal route was developed in the late 1980s and involves a similar technique to that of second trimester amniocentesis. There are various techniques for performing CVS and as there are no studies comparing techniques, the operators should use one that they are most familiar with. Nevertheless, the technique preferred by the author is as follows. It is an aseptic single operator technique, under continuous ultrasound guidance where the operator performs the ultrasound scan and the needling. A 20-gauge needle is inserted percutaneously directly into the placenta being careful not to enter the amniotic sac. The stylet of the needle is removed by an assistant and a 20-ml syringe with 5 ml of heparinized saline is attached to the needle. Negative pressure is then applied to the syringe and the needle moved backwards and forwards in the placenta a few times to suction placen-tal tissue within the needle. While the negative pressure is maintained the needle is then withdrawn and the contents syringed into a sterile container.
The older transcervical approach was developed in the early 1980s. This is now used rarely when the placenta is low and posterior and is not approachable directly by the transabdominal route. This is a 2-operator technique with the patient in the lithotomy position. While one operator scans transabdominally with the transducer in the mid-line, the other operator attaches a tenaculum to the cervix and inserts a 2-mm CVS forceps through the cervix into the placenta under ultrasound guidance and takes a sample.
Meta-analysis of the randomized trials of transcervical CVS yields an excess loss rate in the CVS group of 3.7% compared to mid-trimester amniocentesis . One study reported a significantly lower miscarriage of transabdominal CVS compared to the transcervical , which is then comparable to that after amniocentesis. The transabdominal approach is preferred, not just because it may be associated with a lower miscarriage risk, but also because it avoids putting the women through an uncomfortable procedure in the lithotomy position.
It is now recommended that CVS is not performed before 10 weeks because of the reported association of early CVS and isolated fetal limb disruption and oro-mandibular hypoplasia. However, large series of CVS performed after 10 weeks do not show any increase in the rate of limb defects .
Cytogenetic analysis of the CVS sample is similar to that of amniocentesis. However, placental karyotype may not be exactly the same as the fetus, especially if there has been a post-zygotic non-disjunctive event after cells destined to become the placenta have separated off from those giving rise to the fetus. Confined placental mosaicism occurs in around 1% of chorionic villus samples and will require reanalysis with a second trimester amniocentesis. Mosaicism is only confirmed in the fetus in about 10% of cases.
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