There are two CVS approaches. Transcervical CVS is performed at 10 to 13 wk gestation. Transabdominal CVS may be performed from the 10th wk of gestation to term. It is contraindicated if the bowel or bladder interferes with the needle path or if the skin in the area of needle insertion is infected. Loss rates for transabdominal CVS performed after 12 wk gestation have not been extensively evaluated.
Placental location determines which approach is used.
Low-lying or posterior placentas are usually more easily sampled transcervically. The transabdominal approach is best for fundal placentas or those located anteriorly in a slightly anteflexed uterus and for women with cervical leiomyomas or long, angulated endocervical canals. If CVS is contraindicated, amniocentesis is the alternative. After CVS, fetal heart rate is verified by ultrasonography. Adequacy of all samples is assessed immediately under a bright light or, if necessary, under a dissecting microscope.