Rhogam shot

Hey ladies. I recently got blood work at my 12 week visit and learned today that I am B- and will need the rhogam shot at 28 weeks. I read about it online and some sites are saying it’s not needed until your second pregnancy but my doctor urges that I get it this pregnancy even though it’s my first. Although I’m going to listen to my doctor I am wondering how many of you guys got it your second and not your first pregnancy and did you notice a difference ? My mom was also B- and had to get the shot as well as a cerclage because her body constantly tried to reject me. I have an older brother who she did not have that problem nor the shit with so I’m wondering why ? Does anyone know ?