non-invasive prenatal testing can be used if the mother is rhd-. however, in the case of maternal rhd status being negative, invasive prenatal testing may be used to determine the foetal rhd status instead. the two most common invasive methods of extracting foetal dna are chorionic villus sampling (cvs) and amniocentesis (amc). these invasive procedures can be conducted on both rhd+ and rhd- mothers. after the invasive procedure, medications that prevent the rh immunisation are usually prescribed to rhd- mothers. this is done to avoid the production of maternal anti-d antibodies which may attack the foetal blood cells should the foetus be rh incompatible with the mother.