At the tenth week of pregnancy the embryo is developing inside the sac of membranes, which also contains the amniotic fluid. The sac is formed by twO membranes. The amnion or inner membrane is closer to the fetus and the chorion or outer membrane is closer to the uterine wall. By this time, the chorion has formed a very large number of villi, the beginnings of the placenta. Some villi reach out into the lining of the uterus (the endometrium) at the point where the placenta will soon be located. Others lie loosely free in the endometrial cavity. Once the placenta is fully formed, the villi will shrink and disappear, but at 10 weeks they are growing rapidly. Since the placental and fetal tissues have the same origin, samples of villi can be used to determine fetal genetic disorders.Under ultrasonic guidance, a CVS can be done either transabdominally-through the abdomen-or transcervically-through the vagina and cervix. The trans cervical route is more commonly used, although which route is chosen may depend on the location of the villi, or the preference of the physician performing the test. In the trans cervical route, a narrow plastic tube or catheter is directed through the vagina to the cervix and through the cervix to the chorionic villi. In the transabdominal route, a needle is introduced through the abdominal wall. With either procedure, a small amount of villus material-about the size of a pea-is suctioned into the catheter.

CVS is performed between 10 and 12 weeks of pregnancy. Since the villi are dividing rapidly, some cells may show distinct chromosomes even before they are grown in culture. When this is possible, an immediate diagnosis of an abnormality of chromosome number is feasible and fetal sex can be determined. Generally, however, the cells are put into a culture medium and grown. This allows for chromosome mapping or karyotyping to be readily accomplished.

Results take approximately 10 days to 2 weeks. Fetal sex and fetal trisomies can be identified by CVS. Cultured villi also can be studied for chemical evidence of other genetic disorders.

Parents must decide before they have this test, sonogram, or amniocentesis whether they want to know the sex of the baby. Of course, in a sex-linked disorder like hemophilia, or if there is an abnormality of the sex chromosomes, this is important information. In other circumstances, it is a personal choice. If you decide you do not want this information, you must let your physician or midwife know in advance and inform the person performing the test. You can change your mind at any time during the pregnancy from not knowing to knowing; once the test has been performed, the information is available.

There is little risk to the mother from CVS. Most women feel some pressure or cramping during CVS, but it is usually slight. When the procedure is performed through the abdomen, local anesthesia may be injected at the site where the needle is introduced. A full bladder enables a better ultrasound view of the pregnancy. This may cause some discomfort. A small amount of bleeding may be noted for a few days following the procedure. Most women rest immediately following CVS, but resume normal activities shortly afterwards. The incidence of intrauterine infection is greater than that experienced with amniocentesis, but these infections occur in less than 1 percent of the women having the procedure. They usually respond promptly to antibiotic therapy.

Estimation of risk of fetal loss after CVS varies, but is usually reported as approximately one-half to 1 percent. As with many medical procedures, this will depend on the experience of the person performing the test. The risk is higher than the risk following amniocentesis, and for this reason more women who desire genetic testing choose amniocentesis. Following CVS, you must call your physician or midwife if you have any vaginal leakage of fluid, fever, severe cramps, or bleeding. Some studies have suggested an increased risk of deformity of the fetal extremities following CVS. This may be due to a disruption of the blood supply to the fetal arms or legs. Although studies show conflicting results, the overall risk appears to be slightly increased over the usual expected rates; with CVS the risk of a limb deformity is no more than 1 in 3,000 to 1 in 1,000 procedures. The higher rates occur when the procedure is performed before 9 weeks gestation. For this reason, procedures today are not performed before the tenth gestational week-counting, as usual, from a woman’s last menstrual period. CVS is considered quite accurate. About 98 percent of the time, adequate tissue is obtained. Sometimes, however, the results are inconclusive and an amniocentesis is necessary for clarification.

Amniocentesis is needed when the villus sample shows a chromosomal pattern called mosaicism. In mosaicism some cells show chromosomal abnormalities, while others do not. Mosaicism may not be present in the fetus, although it is seen in the placental tissue. Only an amniocentesis can determine whether the pattern is fetal. Also, CVS does not detect neural tube defects. Since only tissue, and not fluid, is collected, alphafetoprotein cannot be measured by this procedure. Women who have a CVS are still advised, therefore, to have maternal serum AFP testing at 15 to 18 weeks of pregnancy. If this value is elevated, the woman will need a sonographic evaluation and possibly amniocentesis. If the main reason to have fetal assessment is because of an increased risk of a neural tube defect, an amniocentesis is the more appropriate test. Approximately 3 percent of women who have CVS also have an amniocentesis for various reasons. The main advantage of CVS is that it can establish the diagnosis early enough so that abortion can be carried out late in the first trimester or early in the second trimester of pregnancy, which is safer for the mother than a later abortion.