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.