Can the RH Factor harm my baby before the 28th week?

No this will not effect your baby or in fact this pregnancy at all. I am Rh - as well as was given a Rhogam shot at 28 weeks. After the birth of your child the baby's blood will be tested to see if it is Rh + or -. If the blood is negative like yours then you are find (my daughter was O- so I didn't require another Rhogam shot). If the baby has positive blood type then you will receive another Rhogam shot because there is always the chance that your blood mixed during delivery and if the baby's blood is positive then your body will produce antibodies to fight the blood & this could in fact mean that you could have problems with later pregnancies (your body thinking the growing baby is an infection and producing antibodies)