A tongue of tissue within the placenta composed of chorion and containing capillaries connected to the blood circulation in the fetus that laps the mother's blood in the uterus, exchanging oxygen, nutrients and waste products between the fetus's blood vessels (in the villi) and the mother's blood. This tissue is sampled for genetic testing with a CVS (or chorionic villus sampling. See also intervillous space. Plural: chorionic villus.

(CVS) A test done at about 9 weeks pregnancy at which, under the guidance of transvaginal ultrasound, a catheter is passed through the cervix of the pregnant uterus to obtain a small sample of tissue from the placenta (the afterbirth) for genetic testing, such as a karyotype.

Due to trisomy 21. Chromosome 21 is the smallest of the "autosomes" (the non-sex chromosomes): trisomies of the other autosomes tend to be lethal at an earlier stage of embryonic or fetal development, and so are seen much more rarely. Diagnosis of Down's syndrome requires a karyotype, obtainable from pregnancy tissue by chorionic villus sampling (CVS) or amniocentesis. Screening for increased risk in pregnancy can be performed by triple screen or by looking for nuchal translucency at transvaginal ultrasound.

Measurement of alpha fetoprotein in blood serum. High levels can indicate a birth defect involving the brain or spinal cord, such as anencephaly or spina bifida (confirmed if amniocentesis shows high AFP levels in the amniotic fluid). Low levels can indicate an increased risk of Down syndrome (or trisomy 21), trisomy 18 and, occasionally, Klinefelter syndrome, signaling the need for a karyotype of the fetus's tissues by chorionic villus sampling (CVS) or amniocentesis.