Medical Genetics Amniocentesis, a sampling of the amniotic fluid surrounding the fetus is routinely done at 14-16 wks Amniocentesis testing for chromosome disorders is 99. 8 per cent reliable for chromosome number and there is a risk of miscarriage (usually 1/250 or less) after the procedure.Medical Genetics Amniocentesis, a sampling of the amniotic fluid surrounding the fetus, is routinely done at 14-16 wks. Amniocentesis testing for chromosome disorders is 99.8 per cent reliable for chromosome number, and there is a risk of miscarriage (usually 1/250 or less) after the procedure