OP - your initial bloodwork will include a test for RhD sensitivity since you are RhD-. The actual test is called an indirect Coombs test, but I think on the lab paperwork it has another name because they also test for other antibody incompatibilities. If this tests comes back negative, then you are not sensitized, from either your own birth with a possible RhD+ mother or from a previous pregnancy where the baby was RhD+.
If the test shows you are sensitized, your pregnancy will be monitored more closely, as PP mentioned, unless you opt (and pay for?) the early test to determine the in utero baby's blood type. I don't know how invasive this test is though (do they insert a needle? eek!), or if there is any risk for the fetus. I don't know much about the test, I would like for someone to fill us in here!
If the test shows you are not sensitized, the protocol for RhD- mothers with RhD+ partners in the US (used to be?) one shot at 28 weeks, and another within 72 hours after the birth. Additional doses are recommended in certain situations, ie abortion or threatened or actual miscarriage; invasive testing like CVS, amniocentesis, etc; after an external version; etc. The manufacturer of the main formulation used in the US, Rhogam, says that the shot lasts for 12 weeks, although I am not sure where this data came from. So, by this reasoning a 28-week shot and a post-birth shot would provide continuous coverage (up to the limits of the shot) from the 28-week mark onward. It's not quite this simple, since the Rhogam shot is "dosed" to provide protection for a certain amount of exposure to fetal RhD+ blood (up to 15 mL). If a mother experiences blood mixing that is in excess of this upper limit for the dose (for example, trauma to the stomach, car accident, etc), then it is possible that some of baby's RhD+ cells will remain in circulation after the anti-D in the shot has been expended and will cause the mother's immune system to sensitize to the RhD antigen. Also, I believe that some women who are exposed to a large amount of blood mixing may not become sensitized, while others who are exposed to only a small amount of blood mixing do become sensitized. Just as with everything in nature, I believe it is a highly individual and difficult-to-predict process in an individual woman.
According to the University of Connecticut Health Center, ACOG recommends repeated doses of Rhogam to pregnant women every 12 weeks while they remain undelivered. I can't find the ACOG bulletin where the recommendation was actually made, so I don't know when this protocol should start (according to ACOG). But, this may be the source of your mom's recommendations for 14 and 28 weeks (official Rhogam dosing instructions still put 28 weeks as the time of first dose unless there are other factors mentioned above). That would put dosing at every 14 weeks, which is a couple weeks outside the 12-week recommendation. Dosing at (12, 24, and) 36 weeks could lead to the newborn testing positive on the direct Coombs test at birth. The direct Coombs test is a different test from the indirect test. In general it measures if an infant has been exposed to anti-D antibodies, presumably from the mother's blood, and needs a transfusion after the birth. However, if a mother receives Rhogam very near the time of birth, it is possible that the test could come back positive and require transfusion for the newborn, because the Rhogam shot itself can attack (baby's) red blood cells that have RhD antigen on them.
The original data on anti-D (Rhogam) came from experiments that were done on inmates several decades ago. They injected RhD- men with RhD+ blood and then measured sensitization with and without the prophylaxis. This became the basis of the original dosing recommendations and intervals. IIRC, the inmates were injected on a Friday, and they couldn't access them again to test until the following Monday, so that is where the 72-hour rule following childbirth came from. AFAIK there have been no other studies to determine exactly how long a woman has to get the anti-D shot to prevent isoimmunization, and I am sure it is an individual thing. I believe that most physicians recommend that mothers who miss the 72-hour window still receive the shot within 4 weeks of birth, but there is no data to determine how effective the shot at this point would be. IIRC the average rate of isoimmunization for an RhD- woman with a RhD+ baby in the absence of ANY Rhogam is 13% (meaning 87% will be fine and their body will not mount an immune response). For women who receive the post-birth shot ONLY and not the prenatal shot, the incidence drops to 1-2% (98-99% will not mount an immune response). For women who receive the shot at 28 weeks and the post-birth shot, the incidence drops to 0.1-0.2% (99.8-99.9% will not mount an immune response).
Another thing to consider is that the anti-D shot is derived from human blood plasma and therefore has all the risks of a blood product. I don't know if anyone has actually ever had a disease transmitted due to the anti-D shot, but the risk is still there, however small. Also, some people are not comfortable with injecting another person's blood product into their body for other reasons.
It's a complex situation, and I don't think there is one "right" answer for every woman. Rhogam has helped to reduce the rates of hemolytic disease of the newborn since the 60's when it was introduced. I personally opted out of Rhogam completely with my first daughter. I was not sensitized following that pregnancy, but we also do not know her blood type. This time I purchased two Eldon cards to test the blood of the new baby as well as my first child. I believe my midwife will also order a lab test to verify baby's blood type from cord blood. If it is positive, I am not sure what I will decide. I may still forego post-birth Rhogam.
Good luck to you, however you decide.
Edited by Pirogi - 12/30/10 at 10:36am