Here's how I understand it. Once your interviewer turns in their evaluation and your file is complete, it goes to a regional subcommittee based on your undergrad. The subcommittee can reject you or send your file to the main committee. Once you make it to the AdCom, your file is reviewed and you are either accepted right away or they put you back in the pool to be reviewed at the next meeting. But from my understanding, they don't waitlist/reject anyone that makes it to that stage until the end of March.
A friend of mine interviewed at WashU during their last week of interviews last year, got waitlisted the last week of March, then was accepted off the waitlist on May 20th. So there's always hope!
I loved WashU and I'm anxiously waiting to hear back from them. Good luck!