Chorionic villi sampling is the taking of genetic material within in the chorionic villi of the placenta. Chorionic sampling is done when a patient is at high risk for chromosomal abnormalities (previous pos. tests, 35yo or older, family hx.) during the 10-13 weeks of pregnancy.
Confined placental mosaicism results when the C.V.S. testing comes back back showing a trisomy, but all subsequent testing (and the fetus itself) have normal chromosomal counts. This may be due to either a trophoblastic mutation, or by "trisomic rescue," in which trisomic cells that were supposed to be in the fetus are confined to the placenta to prevent an abnormal fetus.