What is the difference between CVS and amniocentesis?

Both are diagnostic tests giving a definitive answer. They differ in timing, in what is sampled, and in one important limitation. CVS takes a small sample of placental tissue, usually from around 11 to 14 weeks. Amniocentesis takes amniotic fluid containing cells from the baby, usually from around 15 weeks. The main advantage of CVS is timing — an earlier answer, which matters if you are considering your options and want to decide sooner. The main limitation of CVS follows from what it samples. Because CVS analyses placental tissue, it shares the same vulnerability as NIPT: the placenta can occasionally have different chromosomes from the baby. Confined placental mosaicism can therefore produce an unclear or misleading CVS result, sometimes requiring a follow-up amniocentesis. Amniocentesis, sampling cells from the baby, does not have this issue. Both can be extended with chromosomal microarray to detect small deletions and duplications, and both can test for a specific inherited condition where one is known in the family. Both carry a small procedure-related risk of pregnancy loss. Ask your unit for their own figures for each procedure, rather than accepting general estimates. Which is right for you depends on how early you want an answer, whether the indication is a first-trimester finding, and your own weighing of timing against the small chance of an ambiguous placental result.