Google pregnancy months to weeks, and click images. It will show you many charts that show this stuff.
The first trimester is weeks 1-13, second trimester is weeks 14-26, and third trimester is weeks 27-40 (that one has the "bonus" week because 40 weeks isn't exactly nine months, it's a week longer). So the second trimester starts at 13w1d, the beginning of the 14th week and the beginning of the fourth month.