Can we not do that? It's worse than saying your 3-yo is "37 months". "The 4th Trimester" was already a stretch, but kind of understandable, as it's immediately after pregnancy/birth. But anything beyond that is dumb.
Is high school graduation the 8th trimester? Walking your child down the aisle to be married, the 12th trimester?
Trimester means 3 measured periods; it's intended to mark sections of a pregnancy. Not every parenting milestone. That's just called parenting.