In response to the question regarding the possibility of two Rhesus negative parents producing a Rhesus positive child. I know this is possible because my parents (Rh neg) have two neg children and my brother is positive (and looks exactly like my dad!). Genetics is a fascinating and complex area of study as is the area of blood grouping. The Rhesus system in humans is not simply a matter of being positive or negative. There are 51 different Antigen types that contribute to Rhesus status. So heredity is dependant on many different contributing antigen combinations.
Put very simply, if two parents are homozygous Rh neg, they are very unlikely to have Rh pos children as they carry the dominant gene for this grouping. But two heterozygous negative parents have a 25% chance of having an Rh pos child since they both carry the recessive gene. But in reality, the combinations are much more intricate.
So make no assumptions about need for giving Anti-D (Rhogam) when both parents are Rh neg. It is presently a blood derived product, so it is theoretically possible for blood borne diseases to be transmitted. But women need to weigh up the risks and benefits themselves and make informed decisions. The risk of complications from subsequent pregnancies without Rhogam is very high and potentially tragic.
-Karen Blake R.M. BMid., New Zealand
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If you don't understand how this could happen, you also will not understand how parents with blood groups A and B can have a child with group O, or why it is impossible for a child to have group O if either one of the parents is AB. Read any basic genetics text or read up on blood groups.
I don't know what the protocols are for RhoGam use in the U.S. as I work in Norway. Here, we only give it to Rh negative mothers without anti-Rh antibodies, and whose babies are Rh positive. We test cord blood so the babies don't get pricked. Two Rh negative parents cannot have an Rh positive child, so one could make a case for testing men and deciding on that basis which women to give RhoGam. There is, however, always the possibility that the biological father of the baby is someone other than the person whom the mother feels compelled to acknowledge officially as the father, and this could lead to failure to offer RhoGam treatment to some women who need it. Testing the baby eliminates this uncertainty. In cases where a woman who is involved with an Rh negative man needs RhoGam after birth, it should be up to her whether to spell this out for her partner, and the injection should be offered in such a way as to protect her right to confidentiality.
Rachel Myr, midwife
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It is strictly impossible for two Rh negative parents to have an Rh positive baby, exactly in the same way that it is impossible for two parents with blue eyes to have a child with black eyes. Rh+ means that the gene exists; Rh- means that it does not exist. Parents can't transmit a gene they don't have.
-Francoise Railhet
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She can be Rh neg if both her parents are heterozygous; i.e. have one pos and one neg gene each. She got the neg gene from both parents, a 1 in 4 chance. Two Rh neg parents cannot have an Rh pos baby (or there would be questions to be answered).
-Phil Watters, Australia
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The woman who asked the question is correct: 2 Rh neg. parents will always produce an Rh neg. baby. (It is worth noting that Rh testing is not 100% accurate, so it would be best if the father had tested negative more than once in his life.) Rh negativity or lack of the Rh factor is a recessive trait. That is, it only shows up if the gene contributions from both parents are Rh neg. It is also entirely possible that someone with all 4 grandparents Rh pos. could end up Rh neg. Someone who is Rh pos. could have EITHER 2 Rh pos. genes OR 1 Rh pos. gene and 1 Rh neg. gene. Thus, 2 Rh pos. parents could produce an Rh neg. child if both parents had a recessive Rh neg. gene, and that was the gene that both parents happened to pass on to the child. Because there are four possible combinations of genes they could pass (++, + -, - +, - -), and 3 of those include an Rh pos. gene, there is a 1 in 4 chance that the child would be Rh neg.
-Sharon Fuller, LM
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It's those recessive genes that are responsible. Yes, your parents could test, test, and test again, and still come up Rh+. What those blood tests don't reveal is the fact that each of your parents carried a recessive Rh- gene. Turning out Rh- requires two Rh- genes, one from each parent, because Rh negativity is a recessive trait. If you receive one Rh+ gene from one parent, and an Rh- gene from the other, Rh+ wins out, and you are what they call phenotypically (the seeable trait that shows up on the surface) Rh+. You are genotypically (the hidden traits you can't see) both Rh+ and Rh-. So the Rh+ parents who each have an Rh- gene have a 25% chance of producing an Rh- child. If you do the Mendelian box to calculate this you can figure it out.
-Kay Jackson, CNM
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Reprinted from Midwifery Today E-News (Vol 2 Issue 5, Feb 4, 2000)
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