How can I reduce the risk of stork-bite birthmarks in my baby?

You can't. If it is in the genes it will show up on the baby. I often see a pattern of storks bite marks in several family members or they may come up as a new finding . I've never had a parent complain about the mark in > 30 years of practice. Given what I have seen over the years i'm just happy when the parts are ale there & seem to fit.