Anyone ever had a Percutaneous umbilical blood sampling(PUBS) also known as Cardocentesis?

This is my third pregnancy and my second run of a gestational condition called Thrombocytopenia. The doctors seem to think this that it is the hereditary condtion called ITP (Idiopathic Thrombocytopenic Purpura). They want to do an Percutaneous umbilical blood sample (cardocentesis) to make sure the baby does not have the same condition. If he does then they will know if a c- section is necessary. Anyone ever had this? Wht is the procedure like? Do they numb you? Does it hurt?