My doctor thinks I cheated

It’s a routine thing for any pregnant women who is a negative blood type (Rhesus-negative, to be specific – I’m not trying to say one blood type is better than another) to be given a RhoGAM shot. I don’t know what RhoGAM means, really, but I know the shot suppresses an Rh-negative mother’s immune system. This is important, because if baby is Rh-positive and Rh-positive blood somehow crosses into the mother’s body at some point, her body will start producing antibodies against that Rh-factor. Then, the antibodies can cross the placenta and destroy the red blood cells in her unborn baby or in the next Rh-positive baby she has. This isn’t particularly good for baby, believe it or not.

You may have figured out that I would not be telling you this if I were not a negative blood type (O-negative, if you’re curious).

Getting the shot was a surprisingly painless process – I just showed a nurse my butt and she stuck the needle in and I was on my merry way – but understanding why I was getting the shot took a little longer.

You see, my husband is B-negative (he says his blood type is also his life motto).

My understanding of genetics is that the positive factor is dominant over the negative one, meaning that a person could have one gene that’s Rh-positive and one gene that’s Rh-negative, and they would still be Rh-positive. You are only Rh-negative if both genes are Rh-negative.

Well, if I’m Rh-negative and my husband is Rh-negative, we both can only pass on Rh-negative genes, so there should be no possibility for this Little Parasite to be Rh-positive.

And yet, my doctor told me to go get the RhoGAM shot.


I sent a text message to my almost-a-doctor friend, asking him whether my understanding of genetics was faulty or whether my doctor was suggesting I may have cheated on my husband.

“Cheated on your husband,” he answered.

“All the infidelity that came before you ruined it.”

But then an interesting thing happened – we were both curious enough to look it up. And we both realized that, in fact, what we had been taught about this particular aspect of human genetics wasn’t quite right.

It seems it’s not just having the Rhesus factor (the one we’re interested in is called “D”) or not – there’s a third possibility: sort of having it. There’s such a thing as having a “weak D” and having a “partial D” which can both be interpreted as their owner being “Rh-negative” in some tests, but Rh-positive in others.  The incidence of “weak D” in Caucasians is about 0.3 per cent, so it’s an unlikely circumstance, but still possible.

That made me feel better – way better. My doctor is just being cautious.

(When I explained this piece of information to my husband, I think he was convinced I was just making up science to cover up my tracks. So now, my doctor doesn’t think I cheated but my husband does.)

Oh, but wait. I read a little more about it, and mistakes made while finding blood type are way more common at a hospital laboratory than at a blood bank. And since my husband has donated blood more than 60 times (what a generous man!), he can be pretty darn sure that he is, in fact, a true Rh-negative.

I’m not sure where that leaves us in terms of who suspects what about my faithfulness to my husband, but I will tell you one thing; this baby had better come out Rh-negative or else science has some explaining to do.


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