She might be confused with the procedure they do to check if bub is aneamic caused from a certain antibodies RH neg women are at risk of if they have not had anti-d in the past and have made antibodies from exposure. You test positive for this when antibodies are tested early in pregnancy. It can be neccessary to do a transfusion on bub prior to birth but this is extremely rare maybe thats what she is calling an amniocentesis. I'd have ask the specific antibody from an Ob at work. It use to be a reason why lots of RH negative women had stillbirths in the dark ages before anti-D was given.

I'd have to go ask as in 5years of midwifery I have only seen it once and the mummy was transferred to a large Sydney hospital for this to be done so we didn't follow her through from her transfer from us. I know it was successful but I don't think bubs Hb (haemoglobin) was tested from the amniotic fluid.