By now, your placenta would be fully functional. It is your baby's life line, the channel by which nutrients and oxygen are extracted from your blood and delivered to your child, while waste products are siphoned off from the baby's end and replaced into your system for removal. It is like a giant filter that does not allow blood from either side to mix, although the occasional cell will pass between either side.

The blood test you would have had previously to check your blood group and rhesus status would be important because the baby's blood group is determined by the genetic make-up from both parents. In cases of incompatibility, when the mother's blood group is O and the baby is an A or B, isoimmunisation can sometimes occur.

Antibodies will be formed in the maternal circulation which then make their way into the baby's circulation and begin to attack its red blood cells. As a result your baby may become anaemic and also, jaundiced at birth.

However, this is not as severe as rhesus disease. This means having a minus behind your blood group instead of a plus e.g. B- as opposed to B+. Rhesus disease is not a problem in the first pregnancy as rhesus negative women would need to be exposed to the rhesus antigen in a previous pregnancy before antibodies can develop. Your doctor may administer anti-D to neutralise the antibodies during pregnancy if you are rhesus negative.

Ref: M19