Subtracting last two created linked record


I am keeping track of an inventory of dogs in foster homes.
We try to weigh these dogs almost every week.
The weigh-ins are recorded in a separate table. That table has weight , dog name and weigh in date.
I want to be able to automatically subtracts the last two weigh in to know if the dog lost or gained weight.
I am assuming it could be done through a roll up column. If so how do I select the only the last two weigh ins
Thank You