When does the 3rd trimester start? I keep getting conflicting info. The pregnancy app I have on my phone says that I'm in the 3rd trimester this week which I know is bogus. Other websites say 27 weeks and others say 28 weeks! Why is this never clear? I had the same problem with trying to figure out when the 2nd trimester starts. Ugh! Either way, I can't believe I only one 2-3 weeks to go before I'm in the last trimester!!!