Help!!!! Sizing

I just bought my first maternity outfit. I am on my third trimester (29weeks) and since it is gonna start cooling off in Canada and I wanted something nicer than leggings for our birthdays. I bought myself 2 pairs of this pants. Pre pregnancy I was always xsmall but bought these pants small... they fit perfectly they are thight but in a good way. They do have some to stretch, but the girl kept reminding me how i'm gonna get bigger this last 3 Or so months and told me to tried medium. I did, they were ok but kind of baggy they felt like that were falling off my butt.

I bought the small sizes. But l am scared they won't fit till the end because I have no idea how much bigger I'm gonna get. I don't want to have to buy bigger later specially this far into the pregnancy.

Should I exchange then for medium?????