Morning sickness causing me to be late for work

This is my first pregnancy and I've been getting very bad spurts of morning sickness right before I leave my house to go to work which causes me to be late quite often. Although I've never not shown up to work because of it, even though I've wanted to many times. I've informed by boss of what's going on but he (of course) doesn't seem to totally understand. I know that it's my responsibility to be on time for my job but I can't drive and throw up at the same time.

I feel like I'm going to get written up or fired because of my constant tardiness. Is there anything I can do about this? Or a way I can stick up for myself if the worst happens?