Hi i visited my family this week and my mom kept smoking around me, especially when traveling in her car. i am 18 weeks pregnant, can this harm my baby?

Not so much: Passive exposure to cigarette smoke is not desired but is likely to have minimal if any effect on your baby. Just plan your life around avoiding the exposure & if asked tell her why. I would also let her know you don't want her smoking around baby when it comes. It does have a clear increase risk of crib death/respiratory illness from that point.