3rd Trimester Question

Tiffany • First time mommy💙🧸

Okay, so I know I can ask my doctor but I forget every time I go in for a check up🤦🏻‍♀️ Does the 3rd trimester start at week 27 or 28 ??

I have two baby apps one tells me 27 weeks the other says 28 weeks. I’ve also looked it up just on google and some websites tell me 27 and others 28 😑 So can someone please enlighten me. Tia!