When is officially 3rd trimester

Nichole • I am a mother to one girl and one boy:) me and my DH just bought our 1st home together in 2014 and we look forward to the future
When does 3rd Trimester actually begin? My doc says 27 weeks and all the charts I find say 27 weeks but I know alot of ppl say 28 weeks so I wasn't sure!!