It stands for Chorionic Villi Sampling; basically they take tissue from the placenta through either an incision in the abdomen or through the cervix. From what I understand, the risks for miscarriage are higher than with an amnio (among other things - try doing a Google search if you’re interested), but it can be done quite a bit earlier than an amnio, so it’s especially appealing to someone who is taking dex and wondering if the baby has CAH or not!