Your blood type and the babies

I don't know exactly what I read. It was a long time ago and I think it said if you have one blood type and your baby has another, it could hurt you? Or something like that. I have my first visit with an OB in 2 weeks, i'll be 15 weeks. Does anyone know anything about this? I guess I have a rare bloodtype that only 8% of the population has or so i'm told. That percentage has probably gone up by now but anyways, I dont know my so's blood type. He doesnt even know. So im curious if anyone has any info on this.