A mismatch between the blood group of a Rhesus (Rh)-negative pregnant woman and that of her baby. In certain circumstances, this mismatch leads to haemolytic disease of the newborn. The Rh system is based on the presence or absence in the blood of several factors, the most important of which is a substance called D antigen. Rh-positive blood contains D antigen, whereas Rhnegative blood does not. The blood type is determined by genes. Rhesus incompatibility results if a Rhnegative woman is exposed to the blood of her Rh-positive baby while it is being born. There are usually no problems during the first pregnancy with a Rh-positive baby. However, the woman may produce antibodies against the D antigen; in a subsequent pregnancy with a Rh-positive baby, these antibodies may cross the placenta and attack the red blood cells of the fetus. A Rhnegative woman can also be sensitized if she has had a miscarriage, abortion, or amniocentesis, in which the fetus’s Rhpositive blood enters her circulation. Rhesus incompatibility is now uncommon because injections of anti-D(Rh) immunoglobulin are given routinely to Rh-negative women during pregnancy and at delivery. They are also given after miscarriage, abortion, amniocentesis, or any procedure that might result in exposure of the mother to fetal blood cells.
Rhesus incompatibility |
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