I'm sure it won't have any effect. Plenty of women smoke throughout their pg and have healthy babies so I'm sure one night of 2nd hand smoke won't do anything. The only thing is it could make you feel sick I can't stand the smell of smoke and when I was pg my smelling was so super sensitized. Having said all that I wouldn't want to be around 2nd hand smoke either - perhaps your parents could smoke outside while you're staying with them?