babyQ. I was reviewing for boards and had a question about blood types and pregnancy.

Let’s say a mother was type AO and father was type BO and the baby ended up being OO blood type. Assuming the mother and baby share blood, wouldn’t the baby produce antibodies against the mothers AO blood and attack itself? I’m just not clear on how the mother can share blood with the baby and both can have different blood types. Maybe I’m forgetting something blatantly simple.

A. That’s a very good question! Before I get to your scenario, a couple quick reminders about fetal-maternal blood circulation and blood group incompatibilities in pregnancy.

You mention the mother and baby sharing blood. Although it seems kind of counterintuitive, mom and baby each have their own blood; there isn’t an open, direct flow of blood between mom and baby. The only times mom’s and baby’s blood mix is during delivery, or (less commonly) if there is some sort of trauma or placental tear during pregnancy.

As far as blood group incompatibilities go, the thing we worry about in pregnancy is mom making antibodies to baby’s blood. Usually, clinically significant incompatibilities involve a mismatch in Rh antigens (baby is Rh positive, and mom is Rh negative). That’s why we give Rhogam to all Rh-negative moms.

ABO incompatibility occurs too, but it is usually not as severe as Rh incompatibility, for a couple reasons:

  1. There is a low density of ABO antigens on fetal red cells
  2. Serum soluble A and B antigens in the fetus neutralize anti-A and anti-B antibodies.

Anti-A and anti-B antibodies are naturally-occurring, by the way (you don’t have to be exposed to A or B antigens to make them – you just automatically make antibodies against whatever antigens you don’t have). The Rh system is different: to make anti-Rh antibodies, you have to be exposed to the Rh antigen first (and, of course, lack it on your own red cells).

Okay. Back to your scenario: mom is AO, dad is BO, and baby is OO. In this setting, mom will automatically have anti-B antibodies – but they won’t have any effect on the baby’s red cells, as the baby does not express B antigens on his or her red cells.

But as you point out, what about baby making antibodies? It turns out that babies don’t start making anti-A and/or anti-B antibodies until about four months after birth…so that is not an issue in pregnancy.

Big thanks to Joe Chaffin, the Blood Bank Guy, for his guidance in this post! If you have any blood banking questions, he’s the guy to go to. Check out his awesome website – tons of useful resources.