What is Rh?
Rh with antibody screening tests the blood for presence of Rhesus factor (Rh).  Approximately 85% of people have the factor, and are known as Rh positive.  If one does not have it, they are Rh negative.  This screening becomes important when a woman with Rh negative blood is exposed to Rh positive blood such as the case of a blood transfusion or sometimes during pregnancy.

If a pregnant woman is Rh negative and her partner is Rh positive, the baby will most likely be Rh positive.   When a mother with Rh negative blood is exposed to Rh factor, her blood becomes sensitized and mounts an attack against the Rh factor as if it were foreign. This causes break down of the baby’s red blood cells leading to anemia, jaundice or on rare occasions a more severe case known as hemolytic disease in the newborn which could be fatal. A mother can become sensitized following a blood transfusion, abortion, miscarriage, ectopic pregnancy, traumatic accident to the belly, external version (for breech), episiotomy and even following amniocentesis or CVS.  This is rarely an issue in the first pregnancy as antibodies have not yet begun to develop.  It is more important in subsequent pregnancies.
Since the 1960s, pregnant women who are Rh negative have been given an injection of Rh immunoglobulin (RhIg, called RhoGAM) that prevents mother’s blood from forming antibodies in future pregnancies of Rh positive babies.

RhoGAM is routinely given to mothers at approximately 28th week of pregnancy. The shot lasts approximately 12 weeks, and is repeated within 72 hours of birth to mothers with Rh positive babies in case there is any blood exposure...

Reasons to Consider Rhogam
-If baby is Rh positive, Rhogam can be given baby’s blood type can be determined from a sample taken from the cord blood at the time of birth. 
-Rhogam is considered an effective shot and has reduced the number of complications and deaths from hemolytic disease. 
-The  shot is considered safe, according to the manufacturer.

Risks of Rhogam
-Rhogam is made from human blood plasma and there is the risk of infection.
-Although the risk is uncommon, it can occur as with any donor blood.
-Side effects to Rhogam include local swelling inflammation at the site, skin rash, body aches and rarely anaphylactic reaction such as hives. 
-The standard Rhogam preparation contains the mercury compound, known as thimerosal. Use the mercury free Rhogam when available.
-According to practitioners who advocate a gentle birth process with minimal intervention and time allowance for the placenta to separate, there is usually no mixing of blood between mother and baby. This means an Rh negative mother would not become sensitized. Traumatic and difficult births increase the likelihood for sensitization. However, practitioners have no precise way to know.

Considerations
In weighing the risks and benefits of receiving donor blood, some women have elected to not have the 28th week shot routinely, and instead choose the mercury-free Rhogam shot within 72 hours of giving birth to an Rh positive  baby, or if there is a  high risk event during pregnancy.  According to midwife Ina May Gaskin, “The problem with routine prescription of prenatal Rhogam is that many babies who are Rh negative like their mothers will be exposed to the drug, and there has been no systematic study of the long term effects of this product in babies.”  In weighing out the risks and benefits it appears that Rhogam has more benefits, I would encourage Rh negative women to have the mercury free Rhogam shot if they give birth to an Rh positive baby, especially if they are considering future pregnancies.