chorionic villus sampling is usually done between the 10th and 13th week of pregnancy, it samples chorionic villi, which are tiny projections of placental tissue. as the placental tissues are derived from embryonic cells, hence, it contains foetal genetic information that can be used to determine the child's rhd status. there are two types of chorionic villus sampling. trans-cervical sampling involves inserting a catheter through the cervix into the placenta to obtain villi, ultrasound is used to guide the catheter to the site of sampling. trans-abdominal sampling requires the insertion of a needle through the abdomen and uterus to obtain placental tissue. local anaesthesia can be applied to reduce pain from invasive procedures.