So when does the second trimester start? There’s quite a bit of variation here. Some people say the first trimester is weeks 1-12, some say 1-13 and some say 1-14. Logically, since “trimester” means 3 months, I think 13 weeks would have to be the closest (there are a few days more than 4 weeks in a month, so over 3 months it adds up to an extra week). But I can also see the sense of going with 14 weeks, since that is only about 12 weeks from conception... But you know what? I don’t really care any more! For me, the first trimester ends when the placenta takes over and I start feeling more normal again!