I have a question about birthmarks?

I've had birthmarks all my life I was often teased now I'm told they can be dangerous!! they all are light brown in color, I have one on my stomach that is strangely identical to one my grandmother says my mother had. I also have one over my hand. ((one on each hand but the one on my left is smaller)) (also like my mother but bigger on my right hand) but in total I have a little over ten...I was often asked if I burned myself or something. Is this something bad?