Most hospitals and caregivers will now perform a blood test on women (with a negative blood group) in the hours following the birth, as well as taking cord blood for the baby. The test for the mother is called a 'Kleihauer' test (named after the person who discovered it) or a 'Quantative Feto-Maternal Haemorrhage' test (or QFMH). The test measures exactly how much of the baby's blood (if any) has passed into the mother's blood during the birth.
The amount of baby's blood in the woman's system can vary, ranging from 0.1ml to 30mls. Knowing the amount can help better determine how much anti-D Immunoglobulin will be needed to prevent the formation of antibodies in the woman's blood. It is estimated that more than 50% of women will have amounts of 0.1ml or less in their blood, but it is possible for the amount to reach up to 30mls (in 0.6% of cases).
The amount of blood that passes to the mother can be increased by interventions such as Caesarean birth or
manual removal of the placenta. This is because the physical action of the caregiver manually separating the placenta from the wall of the uterus can push more blood into the woman's system, than if the placenta separates on its own.
The amount (or dose) of anti-D Immunoglobulin needed is directly related to the amount of blood that actually passes from the baby to the mother. Caregivers know that 25 mg will protect against about 2mls of the baby's blood. In Australia the standard dose of anti-D Immunoglobulin after the birth is 125 mg (covering up to 10mls of baby's blood), which is generally adequate for up to 99% of women. A Kleihauer test for the woman would show if more than this dose was needed.
In the past when anti-D was plentiful, the standard dose was 300 mg, enough to cover up to 24 to 30 mls of baby's blood in the mother's system. Now because of the high cost and low availability of anti-D, many countries try and give the lowest dose possible to prevent the formation of antibodies.