Rhesus factor

When you have your pregnancy booking appointment your midwife will ask to take some blood samples, one of these will be to find out your blood group.  It will determine whether you are a positive or negative blood group.  If it is found that you have a negative blood group you will be offered Anti-D at different points in your pregnancy.  Anti D is derived from plasma collected from donors, it is a concentrated form of antibodies.

A new test is now offered to all women who are Rhesus Negative at 16 weeks gestation.  The blood test will determine whether your baby has a positive or negative blood group. If your baby is found to have a negative blood group then you will not require Anti D at any point during pregnancy.  Your baby’s blood group will be tested at birth by taking a blood sample from the umbilical cord to confirm they are Rhesus Negative.

When a mother has a negative blood group and the baby has a positive blood group there is a risk of baby developing Rhesus Disease.  This is when the mother develops antibodies (infection fighting molecules) to the baby’s positive blood group in an attempt to destroy them.  The next time the mother’s blood comes into contact with rhesus positive blood her body will quickly release these antibodies as a defence, therefore it will be the subsequent pregnancy that is affected, not the current one. This only happens if the mother is exposed to her unborn baby’s blood. During pregnancy a mother and baby’s blood circulation are completely separate but there are times when there is a risk of the mother being exposed to baby’s blood. These are called ‘potentially sensitising events’ and may happen if the mother has any trauma to her abdomen, such a fall or a car accident or experiences any PV bleeding. Birth is also classed as a potentially sensitising event.

In order to prevent the mother from creating antibodies, Anti D is offered at 28 weeks of pregnancy and postnatally. 

For more information please visit Rhesus disease - NHS (www.nhs.uk)