Rh negative mom / Rh positive dad…Is this a problem?

The four major blood types a person can have are A, B, AB and O. Blood types are established by the types of antigens on the surface of red blood cells. When we talk about blood types we first say the person is one of the aforementioned blood types, but there is another type of antigen that is sometimes found on the surface of red blood cells called the Rh factor. Rh comes from and abbreviation of Rhesus because these antigens were first studied in the Rhesus monkey. If the blood lacks this antigen we say the person is Rh negative. If the blood cells have the Rh antigen, the person is Rh positive.


The main risk for an Rh negative baby born of a Rh negative mom is that the fetus becomes sensitized when the blood of the fetus mixes with the mother’s blood. This can cause an immune response in the fetus called Hemolytic Disease of the Newborn (HDN). The mom’s immune system attacks some of the unborn baby’s cells and destroys them. This destruction can make the baby very sick or even die and if steps are not taken.

It is interesting to note that this immune response only happens in an Rh positive baby of an Rh positive mom if the blood from baby and mom mix. This occurs most commonly during childbirth, so the second child of the Rh negative mom and Rh positive dad has more of a chance to get HDN than the firstborn.

Fortunately, in the 1960’s an intramuscular injection was created called RhoGam. This injection is composed of IgG antibodies. These antibodies can cross the placenta so even though the fetus’ Rh factor is not known, this injection is administered as part of routine antenatal care at 28 weeks.

At birth, a cord blood sample is taken from all babies born to negative moms and they are tested for the antigen. If the baby is negative no further steps need to be taken.

RhoGam is also administered when an Rh negative mom has suffered a miscarriage or has had a stillborn baby and the fetus’s blood type is undetermined.

Problems can occur if the baby is Rh positive and the mother is not, but at present, in most cases, problems can be prevented with medication.

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