Australian researchers endorse non-invasive fetal blood test

A group of Australian researchers are one step closer to a non-invasive test to predict fetal blood group typing for Rhesus (Rh) D-negative women to determine the risk of haemolytic disease of the newborn (HDN)—a condition that can cause fetal anaemia, hydrops fetalis and perinatal death.

The study, published in the July edition of the Medical Journal of Australia (MJA), tested blood samples taken from 140 RhD-negative pregnant women from as early as 12 weeks gestation.

Using real-time polymerase chain reaction (RT-PCR) techniques, fetal DNA present in the maternal circulation was analysed for the fetal RhD blood type.

Results were assigned for 135 samples, all of which were accurately confirmed by newborn cord blood RhD serotyping.

According to the research, RhD-negative pregnant women with a heterozygous partner have a 50 per cent chance of having a RhD-negative fetus that is not at risk of HDN.

Dr Glenn Gardener, the study’s principle clinical investigator and Mater Mothers’ Hospitals Acting Director of Maternal Fetal Medicine, said these women could be reassured and managed less intensively if this could be confirmed.

“Until recently, genotyping to assess fetal RhD status has only been available by amniocentesis which carries a risk of pregnancy loss, in addition to the risk of increasing maternal antibody levels in affected pregnancies,” he said.

Although preventative measures including postpartum and antenatal anti-D immunoglobulin have been introduced for RhD-negative pregnant women, a small number continue to become sensitised and express antibodies to RhD and run a risk of HDN. Babies born with this condition are at significant risk of neonatal morbidity and mortality.

Dr Gardener said he hoped the test would eventually become available to all RhD-negative pregnant women but would initially be targeted at high-risk isoimmunised women.

The research is the published work of Catherine A Hyland, Glenn J Gardener, Helen Davies, Minna Ahvenainen, Robert L Flower, Darryl Irwin, Jonathan M Morris, Christopher M Ward and Jonathan A Hyett.

Participants for the study were drawn from Mater Mothers’ Hospitals and Royal North Shore Hospital, with testing undertaken by the Australian Red Cross Blood Service (Medical Journal of Australia 2009; 191 (1): 21-25).

The Rhesus factor

The two main forms of blood classification include the ABO system and the Rhesus (Rh) type system characteristics that are inherited independently.

Blood is either RhD-positive or RhD-negative depending on whether or not the RhD antigen is present.

An individual who is RhD-negative can develop an immune response if RhD-positive blood gets into their bloodstream. This can happen during pregnancy or delivery if an RhD-negative woman is pregnant with an RhD-positive baby. If red blood cells from the baby travel across the placenta, the woman’s immune system will see the RhD-positive red blood cells as foreign and mount an immune response against them. Specialised immune cells will make antibodies designed to neutralise the RhD-positive red blood cells.

If the woman subsequently conceives another RhD-positive baby, antibodies can cross the placenta and destroy the unborn baby’s red blood cells. If the process is severe, the baby can have such a low blood count that it may die unless it receives a blood transfusion. This is called haemolytic disease of the fetus and newborn (HDN) and can occur when there is an incompatibility between the blood types of mother and baby.