Still, the trimesters of pregnancy can be a very useful “shorthand” of sorts that we can use to talk about the symptoms and the progress of pregnancy. The first trimester starts with conception, and concludes roughly 13 or 14 weeks later. The second trimester begins when the first trimester of pregnancy ends. The second trimester of pregnancy begins at around 14 weeks of pregnancy. The second trimester will last for another 13 or 14 weeks, at which point the third trimester will begin. So, the second trimester encompasses the middle part of your pregnancy, and will lst from around the third month to around the sixth month of pregnancy.