I have crowded teeth and they have some mobility. my orthodontist has said it is caused by pressure from crowding but with my regular brushing, it will not ever fall out. should my teeth soon enough tighten a bit after i get braces and an expander?

Malpositioned teeth trap food and bacteria more so than teeth that are properly aligned. Orthodontic care together with proper brushing, flossing, and professional hygiene visits should resolve your problem.

Crowded teeth are harder to clean. And it's unremoved bacteria that cause bone/gum loss. and with bone/gum loss comes tooth mobility and eventually tooth loss. Advisse that you see BOTH a specialist Periodontist and a specialist Orthodontist for evaluation. "Pressure from crowding" does not cause bone loss.