I know this sounds like a stupid question but when does the second trimester actually start? I know pregnancy is measured in completed weeks (e.g when you are 4+2 you have completed 4 weeks and two extra days so are in the 5th week of your pregnancy). Therefore does it start when you are in the 13th week (so 12+0) or when you are 13+ weeks? My ff app seems to think I will begin my second trimester on Sat when I will be 12+0 but I've heard a lot of people say that it doesn't start until the following week? I realise that it doesn't really make any difference, I am just curious as to what people's opinions are?