Some information from NCT website
People whose blood is rhesus positive have a substance known as D antigen on the surface of their red blood cells. Rhesus negative people do not. About 15% of women are rhesus negative. This isn't usually a concern for a first pregnancy, but it may mean some extra care is needed to avoid problems if you get pregnant again.
A rhesus negative woman can be pregnant with a rhesus positive baby if the baby's father is rhesus positive. If any of the baby’s blood enters the woman’s bloodstream, the woman’s immune system can develop antibodies (infection-fighting proteins) against the rhesus antigens. This is known as sensitisation. A transfer of blood can occur during birth, or if the woman has a bleed or an injury.
Production of the antibodies is not a problem in a first pregnancy, but when the woman is pregnant next time with a rhesus positive baby, her antibodies can attack that baby’s red blood cells. This can result in a serious condition called haemolytic disease of the newborn, which leads to anaemia and jaundice in the baby.
If the woman is given an injection of a solution called ‘Anti-D’, it will ‘mop up’ any rhesus positive antigens, preventing production of antibodies against the baby. Anti-D injections reduce the risk of a rhesus negative woman becoming sensitised.
NICE recommends routine antenatal administration of Anti-D to all rhesus negative women in case sensitisation occurs. This can be given as a one-off dose at 28-30 weeks or as two doses at 28 and 34 weeks. It is quite safe for both the mother and the baby.