Third trimester starts when?

Am

Hi went on to <a href="https://play.google.com/store/apps/details?id=com.glow.android.nurture">nurture</a>

and it told me today is my third trimester. I'm 26 weeks for some.reason I was thinking it's not till 27 weeks. Is <a href="https://play.google.com/store/apps/details?id=com.glow.android.nurture">nurture</a> correct?