Trimester question

Devin • Hello all, my name is Devin and I am new to here. I don`t have much to say other than it will be nice chatting with you all. 😊😘💋❤
Ok, I am very confused. My OBGYN told me that the 14th week is when the second trimester starts. But my app is telling me the 13th week is.... Which is it?