I'm Rh- and both my boys are rh+. I got a blood test done with both babies early on to see if there were any antibodies in my blood from maybe a previous miscarriage I was unaware of. I also got a shot during my pregnancy and after delivery (if the baby was +) to prevent any complications with future pregnancies.

From my understanding the issue would arise if you didn't know you'd had a miscarriage and never got a Rhogam shot right afterwards, or if you refused to get the shot (like some people do) and had an issue where the blood mixed (injury etc) or if you refused or forgot to get the shot after delivery where it's likely blood mixes then you could have issues with your next baby if they were positive.

It's very rare that these problems arise - especially if you take the precautions and get the shot, but even for those who never do like my grandmother who had 7 kids without one rhogam shot - it's still a small percentage that anything would even happen.

Very very sad story, I can't even imagine the pain she'd be feeling!