How long is a trimester? One trimester is 13 weeks. For example, the first trimester = the first 13 weeks. The 2nd trimester is up to 26 weeks. The 3rd trimester is from 26 to the end. The total length of pregnancy is about 40 weeks which are divided into 3 trimesters. That is counting from the first day of the woman’s last menstrual period (LMP).
https://www.acog.org/-/media/For-Patients/faq032.pdf