You cannot usually determine the blood group of your baby until he or she is born. If the woman has a negative blood group, your caregiver will take a sample of blood from the umbilical cord attached to the placenta, after the baby is born and the cord is cut. (This can be facilitated for women giving birth at home or in a birth centre as well as delivery suites). The blood is tested to determine your baby's blood group and Rhesus factor. Women with a positive blood group do not need to have a sample of cord blood routinely taken.
The cord blood sample (about 5 to 10 mls) is sent to the pathology laboratory to determine the baby's blood group and Rhesus factor. Depending on the time of day, the result is normally available within a few hours. If the baby's blood group is positive (for example A positive, O positive etc.) then the woman will require an injection of anti-D immunoglobulin within 72 hours of the baby's birth. This is aimed at preventing the formation of antibodies in her blood. If the baby has a negative blood group, the same as the mother, no injection will be required.
For women with a positive blood group it does not matter what blood group the baby is, because her system can accept both positive and negative blood without stimulating a reaction. For this reason the taking of cord blood and the administration of anti-D Immunoglobulin injections after the birth is not an issue.
If you are having more than one baby, all the babies' cords will need to have samples taken. If at least one of the babies has a positive blood group, then the woman will need an injection of anti-D immunoglobulin.