My OB goes with 26w6d as the start of 3rd trimester.smileymama2bee said:I know this app says third trimester starts at 28weeks but I've been told 27weeks. It's what my doctor goes by. Some ladies are going by 27weeks and others 28. But if each trimester is 13weeks long and there's 40weeks all together. That would make the second trimester start at 13w3d and they hire start at 26w6d. Here's a chart I found along with 98% of other charts saying 27weeks so what do you think?