When does the third trimester start?

Bree 🖤♉️ • Mrs. W 5.5.16 💍 Boy Mom 11.6.17 💙 Girl Momma 8.5.19 💗 Girl Momma 9.22.21 💜 Dog Mom 🐶

Someone clarify for me. I always get confused 🤷🏽‍♀️