amniocentesis is another invasive procedure which can be used to collect foetal dna samples. this procedure is usually done between the 15th week to 20th week of pregnancy. the purpose of amc is to extract a small amount of amniotic fluid as foetal cells may be shed from the foetus and are suspended in the amniotic fluid. the foetal genome can be found in these cells. therefore, extracting amniotic fluid can the required foetal genetic material for the genotyping of the rhd gene. before amniocentesis commences, the doctor will inject local anaesthetics to the mother's abdomen. the doctor will then apply ultrasound to locate the foetus in the uterus. under the guidance of the ultrasound imaging, a long, thin, hollow needle will insert through the skin of the abdomen to the uterus of the mother. the needle is used to withdraw a trace amount of amniotic fluid. it will then be removed from the maternal body and the amniotic fluid extracted will be sent to the laboratory for further testing.