Let's take Alice and Bob, both healthy adult human specimens. Alice is (A-). Bob is (B+).

If they decide to have a child, would Alice's pregnancy involve any additional risk or complications compared with, say, Abraham and Abigail's pregnancy (both AB+)?


I overheard claims that pregnancies from parents with different blood groups can lead to complications in pregnancy due to interactions between the mother and the child's blood. (These claims were not officially backed, otherwise I would have brought them up on Skeptics.SE). The explanations and my Google browsing went beyond my biology knowledge. I know these pregnancies must be viable, since the AB group exists, but I do not know if there are edge cases that lead to complications that would not happen with same blood type parents.

Related question

Can the mating of a couple with Rhesus positive and negative blood types result in miscarriage? This question only asks about Rh+ and only if it leads to miscarriage.

  • 1
    $\begingroup$ Thanks, @AlexDeLarge. I overheard claims that pregnancies from mixed parents can lead to complications in pregnancy due to interactions between the mother and the childs blood. (This claims were not oficially backed, or I would have taken it to Skeptics.SE) The explanations and my Google browsing went beyond my biology knowledge. I know these pregnancies must be viable, since the AB group exists, but I don't know if there can really be some significant (>0.1% maybe?) edge cases that lead to complications that wouldn't happen with same blood type parents. $\endgroup$
    – xDaizu
    Commented May 31, 2017 at 10:06

2 Answers 2


The rhesus factor (the + / - part of the blood group) can cause complications with pregnancy under certain conditions and lead to the so called Rh disease:

If the mother is rhesus negative and the father, rhesus positive; then the birth of a child with Rh+ blood group (inherited from the father) can cause the mother to build antibodies against this blood group. Therefore, a second pregnancy with a child that also has Rh+ blood group can have complications if no precautions are taken at any other point.

If these specific conditions are not all met, then no complications should arise from the blood groups.


The only complications that will occur will be in the birth of the second child, and that too if the second child is conceived soon after the birth of the first.

Let me explain.

Rh factor is an antigenic protein found on the surface of the RBCs of some individuals( rather, 'most individuals').

enter image description here

If the factor is present, the blood group is written with a positive(+) sign, and if it is absent, the negative sign(-) is used with the blood group. Therefore, in your example, Bob, Abraham and Abigail have the said factor on their RBCs whereas Alice does not.

As a matter of fact, our body has a mechanism because of which it produces antibodies against foreign antigens. Antibodies are simply substances released by our specialized cells to counter toxins and foreign antigens. These antibodies bind with the specific antigens (against which they are released) to render them harmless.

enter image description here

The fact is that these antibodies, once produced in our bodies, remain in the bloodstream for a certain interval of time, and there presence protects us from future infections by those antigens(This is the principle exploited in vaccination).

Let us relate this knowledge of antibodies and antigens to the Rh factor. Let's say that we make a blood transfusion from an Rh positive person(like Bob) to an Rh negative person( like Alice). Since for Alice, the Rh antigen is a foreign antigen(because it is naturally absent from her blood), she will get sensitized and develop antibodies against it. However, these antibodies will not attack the antigens immediately. They will come into action only if the antigens are again brought into the bloodstream, i.e. another transfusion is made sometime later. These antibodies will then bind to the RBCs carrying it, and result in agglutination, or clumping of RBCs. The clump of RBCs thus formed blocks the passage of blood to the heart, resulting in death.

enter image description here

So, if Alice and Bob conceive a child together for the first time, the fetus will be Rh positive(because if the factor is present in any one parent, it will also be found in the child), and it will sensitize the blood of the mother. At this point, if you know some basics of biology, you will think that since the blood of the mother and the fetus never comes in contact, the fetal blood should not be able to sensitize the mother's blood. If you think so, you are absolutely right! The placenta does not allow their blood to mix. However during the parturition, rupturing of placenta often occurs, causing just a small amount of the fetal blood to mix with that of the mother. Even this small amount is enough for the mother to be sensitized and develop antibodies against Rh factor.

Now, if the second child(who will also,obviously, be Rh positive) is conceived soon, there will be a problem. The placenta is permeable to antibodies, and as a result, the anibodies developed earlier will enter the fetus and cause agglutination in the fetal blood, leading to excessive anaemia or even death. This is called erythroblastosis fetalis, or hemolytic disease of the newborn(HDN)

Thus, you can see that Alice and Bob will have a problem with their 2nd child, whereas Abigail and Abraham will not, since they are both Rh positive and Abigail won't be sensitized.

FACT The Rh factor was first found, and named after, the common Rhesus monkey!!

Rhesus Monkey


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