Mothers blood/fetal blood question!

Kal

I have Rh- blood meaning I need the Rhogam shot at 28 weeks Incase my baby’s blood is Rh+. I know this is because if she has positive blood & our blood mixes, my body can reject and fight against hers. My question is, if there is traces of fetal blood in the mother’s blood starting as early as 9 weeks, why do they wait so long to give Rh- mothers the rhogam shot?