Yes, having the RH negative factor in your blood. When I got pregnant I had been in a slow fade for about a year, so still had some influences on me. I found I had the RH factor which usually is not a problem for the first child, but could be in later births, so it is routine to give the mother a RhoGAM injection immediately after giving birth. Not having this could lead to blood related problems for the baby in later births. I found out that RhoGAM is a sterilized solution made from human blood and therefore was forbidden by the Witnesses. Well, since by then I knew what I had been raised in for 25 years was a load of hooey, I had the shot without broadcasting it to my family. (I was remarried to a nonwitness so it really wasn't an issue to us.) The RH negative factor is rare but not uncommon, so I am sure other Witness women have had to deal with this. The only personal experience I knew of was some 50 years ago in our congregation when a woman gave birth to her 3rd child and this was an issue. They wanted to replace the baby's blood with a transfusion which was of course refused. Fortunately the child survived, but had many expensive medical issues that could have been prevented. (She has since grown up, had babies of her own, and last I heard was no longer a Witness!)
And, BTW, to maisha, yes, there are numerous whackos in the congregations when it comes to health care and trying to impose their ideas on others. And the elders telling you that you can't do something to stumble someone else, even if you and your conscience have no problem with it. I was "in" from 1961 till 1984 and saw this all the time.