It's because with an rh blood type
it can be harmful for the baby if he/she has a positive blood type
and they don't want the baby to get sick. They told me as long as I got the shot around the 28th week the baby will be just fine. Also, there weren't any side effects for me at all with the shot just a poke in the bum haha. I read a few articles on how it could be dangerous but the doctor said that it was false, millions of women get the shot.