When does the third trimester start?

Bree πŸ–€β™‰οΈ β€’ Mrs. W 5.5.16 πŸ’ Hunter’s Mom 11.6.17 πŸ’™ Jordan’s Momma 8.5.19 πŸ’— Dog Mom 🐢 Soon to be Cat Mom 🐱

Someone clarify for me. I always get confused πŸ€·πŸ½β€β™€οΈ