The blood type and Rh status don’t hold much importance until you are pregnant. But as Rh incompatibility often leads to various pregnancy complications, the Rh factor screening, either positive or negative, becomes extremely important during pregnancy.

Rh (Rhesus) factor is a protein present on the surface of the red blood cells. If the cells are coated with the protein, the status is termed as Rh-positive, whereas, its absence is termed as Rh-negative.

Rh incompatibility refers to the situation where a child is born to a couple having opposite Rh factors. The risk arises when a Rh negative mother conceives from a partner who has a positive Rh factor. This combination usually produces a child that is Rh positive.

It is important to note that the Rh incompatibility does not pose any threat to the baby during the mother’s first pregnancy, as the chances of baby’s blood entering the mother’s system are rare until delivery. The risk arises during following pregnancies.

As the different blood systems of the mother and the baby intermingle, the mother’s immune system comes into action and it actively creates antibodies against the Rh factor, thus treating a Rh positive baby like an intruder in her body - the process known as sensitization.

As the sensitized mother starts creating antibodies they attack an Rh positive baby's blood, causing it to break down the red blood cells of the baby, resulting in mild to serious anemia in the baby. In severe cases this cell breakdown can even permanently cause brain damage or death.

Sensitization can also occur if an Rh negative woman has had a previous miscarriage, induced abortion, an ectopic pregnancy. Certain procedures like amniocentesis and blood transfusions can also trigger sensitization.

Fortunately, Rh-sensitisation can be avoided by taking proper care and undergoing regular monitoring during pregnancy.

Also, as prevention of development of Rh antibodies, the key to protecting the fetus when there is blood incompatibility, at 28 weeks an injection of a drug called Rh immune globulin is administered. This is a naturally occurring blood product that helps prevent an Rh negative mother’s antibodies from being able to react to Rh positive cells of the baby.

Another dose is administered within 72 hours after the delivery, if the baby is adjudged Rh positive.