My blood type is o- with rh factor, the father/s blood type is a+.HOW CAN MY CHILD BE AB-? HE IS MY FRIST BORN CHILD

Ok, I only did a little research on this but from everything I'm finding it's not possible for your child to have AB when you have O & your husband has A. Here is a link to a website where I found it the easiest to follow. They actually give a chart showing the possible blood type for baby's to have when combining the parents' blood types. From the way the chart has it, the only way possible for a baby to have AB blood type is if either -
1 parent has A & the other B or if 1 or both parents have AB... it also shows that if either parent has O blood type, AB is not possible. So I'm thinking you have some wrong info here about your baby's blood type. Anyway, hope this helps & I'd be on the phone with the doctor ASAP Mon morning to find out what's what.

Understanding Genetics: Human Health and the Genome

Here is more info. There are other possibilities as well.

Please read this.

Understanding Genetics: Human Health and the Genome