Not everyone's blood is the same. Blood is classified into groups, the most well-known being the ABO system in which a person’s blood is recorded as either A, B, AB or O, depending on the types of chemicals identified in their red blood cells. When someone needs a blood transfusion it is important that they are transfused with blood from the same group as their own. Otherwise a reaction to the ‘foreign’ blood may occur.

The rhesus factor

Each blood type is also further identified by a plus or minus sign after the letter of the alphabet. For example, someone's blood may be ‘O positive’ (written O+) and another ‘AB negative’ (written AB-).

The plus or minus sign refers to the presence or absence of a substance in blood known as the rhesus factor, so named because it was first discovered in rhesus monkeys.

Most people are rhesus positive (Rh+). But if a rhesus negative (Rh-) person receives Rh positive blood, their body reacts, making chemicals (antibodies) to defend against the foreign rhesus factor. This is similar to the way we make antibodies to various viruses such as rubella (German measles) and chickenpox.

The rhesus factor and pregnancy

During pregnancy, or at birth when the placenta comes away from the wall of the uterus, some blood cells from the baby’s circulation sometimes make their way into the mother’s bloodstream.

This is normal and for most women not a problem. However, it becomes significant for women who have Rh negative blood. If the baby has Rh positive blood inherited from the father, and the mother and baby’s blood cells become mixed during gestation or delivery, the mother’s body may treat the baby’s blood cells as foreign substances and produce antibodies against them (Rh antibodies).

This doesn’t often cause problems during a first pregnancy, because there’s usually no significant contact between the baby’s and mother’s blood until the baby is born.

However, if she has a second baby who is also Rh positive, then it is possible that Rh antibodies from the mother’s blood will move across the placenta and enter the unborn baby’s bloodstream. These antibodies will then bind with the baby’s Rh positive red blood cells, causing them to be destroyed.

As a result of this, the baby may be born seriously ill, and unless a blood transfusion is given shortly after birth, the baby could die.

In each subsequent pregnancy the mother becomes more sensitised to Rh positive blood and produces antibodies earlier and earlier in each one. In severe cases, the baby may die before birth if a large amount of blood cells are destroyed.


This lack of compatibility between a mother's blood and that of her baby may sound worrying but, fortunately, medical science has developed a method to ensure the problems resulting from incompatible blood are minimised.

Doctors give an injection of Rh antibodies, in the form of ‘anti-D’ immunoglobulin, to a mother who has Rh negative blood within a few hours after she has given birth to a baby with Rh positive blood. This destroys any Rh positive blood cells which have been transferred to her from the baby, preventing her from producing antibodies that might harm future babies. Like all vaccines, ‘anti D’ is not 100 per cent effective in all cases, however, it can help protect the health of future pregnancies for many women.

To prevent early sensitisation, doctors also give women with Rh negative blood an anti D injection at 28 weeks of pregnancy and again at 34 weeks, as well as after the birth of her baby.

Earlier or additional doses of anti D are also generally given if there is an episode of vaginal bleeding during the pregnancy, and when invasive tests such as amniocentesis or chorionic villus sampling are performed.

Women who have a miscarriage, an ectopic pregnancy or a termination of pregnancy will also need anti D, even if it is the first pregnancy, to protect future pregnancies.

Sometimes a woman’s Rh antibody levels need to be measured periodically during her pregnancy to anticipate whether the baby might have problems. If her antibody levels are too high, then she might need further tests to check the health of the unborn baby. Sometimes the unborn baby needs a blood transfusion, or it might need one soon after birth.

On your first visit to a doctor during your pregnancy, you will usually have your blood type checked so that the problems described above can be avoided or minimised.

Last Reviewed: 05/08/2009