Alloimmunisation against the RhD red cell surface antigen was the most common cause of haemolytic disease of the fetus and newborn until recently. Maternal plasma has now almost replaced fetal cells obtained by amniocentesis or chorionic villous sampling, as the source of fetal DNA, hence eliminating the need for invasive sampling procedures. The fetal rhesus typing was done from maternal plasma in a woman in her fourth pregnancy, though she was isoimmunised in her previous pregnancies and needed invasive tests like cordocentesis in the past. The fetus was diagnosed to be rhesus negative from maternal plasma and that avoided the need for amniocentesis or cordocentesis.
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