So I'm confused. My specialist says that 12 weeks is the end of the first trimester, and my nurture glow app says that also. But my baby center app says the first trimester doesn't end until 13 weeks?

I'm just curious because I will be 12 weeks tomorrow and my specialist is having me stop my progesterone suppositories cuz she said it's the end of the first trimester and I don't need it after that. My nurture glow app also says that 12 weeks is the end of the first trimester. But my baby center app says the first trimester isn't until the end of 13 weeks? Which is correct? I'm just nervous to stop my progesterone and hope that my placenta is actually taking over already