Researchers in Canada and the Netherlands are reporting on a Doppler ultrasonography technique that can safely and non-invasively detect anaemia in foetuses. The procedure is not only better than traditional testing methods, which usually require invasive amniocentesis, but also far less traumatic to the mother and foetus (New England J. Medicine 355 156)
Anaemia occurs when there are too few red blood cells in the bloodstream, which results in insufficient oxygen reaching tissues and organs. In foetuses, the condition generally arises because of complications from Rhesus disease, which occurs when a mother has Rh-negative blood and the father has Rh-positive blood. If the foetus is Rh-positive, the mother's immune system will produce antibodies that attack the red blood cells of the foetus.
Although not usually a problem during a first pregnancy, the condition can be fatal (to the foetus) in subsequent pregnancies with a Rh-positive baby if the mother is not given medication to counter the immune response. If the mother's blood has already developed antibodies to Rh-positive blood from a previous pregnancy, they will try to destroy any Rh-positive blood cells that they meet. And if enough foetal blood cells are destroyed, this can lead to anaemia, heart failure, severe jaundice and even death of the foetus.
Doctors can monitor the baby's condition in the womb, and so perform in utero blood transfusions if the anaemia becomes severe. Currently, this is done by repeated amniocenteses throughout the pregnancy. However, this is an invasive technique that involves inserting a needle into the amniotic cavity. Not only is it uncomfortable for the mother but also it carries risk of miscarriage as well as a risk of worsening the foetal anaemia by further mixing the baby's and mother's blood.
The new Doppler technique overcomes this problem since it relies solely on ultrasound interrogation. To diagnose foetal anaemia using this method, doctors can simply measure the speed of blood flow through a region of the foetus's brain called the foetal middle-cerebral artery. Foetuses with anaemia have thinner blood, which moves faster through the brain than blood with a normal number of red cells.
To evaluate their technique, Gareth Seaward from the Mount Sinai Hospital in Toronto, Canada, and colleagues at the Leiden University Medical Centre, the Netherlands, studied 187 pregnant women who had evidence of Rh antibodies. The volunteers underwent both amniocentesis and Doppler ultrasound. The researchers found that the accuracy of ultrasound was 85%, compared with 76% for amniocentesis. The ultrasound technique is therefore better for detecting foetal anaemia than an invasive test and does not expose the foetus to any risk, says Seaward. In Canada, amniocentesis causes miscarriage in about one in every 200 pregnancies.
Seaward adds that the new technique, which is available on most modern ultrasound machines, would be practical in dedicated high-risk foetal-medicine units. He believes the research will eventually allow doctors to follow foetuses at risk of anaemia through the Doppler ultrasonogaphy technique alone.
The team now plans to determine whether serial measurements of foetal middle-cerebral-artery Doppler can predict when a foetus is likely to develop severe anaemia.