When does the second trimester end?

26.5 weeks. See answer to when does 1st trimester end and do the math.
U can look this up. But 26 2/3 weeks, each trimester is 13 1/3 weeks as 13 1/3 x 3 trimesters equals 40 weeks and a piece of advise do not let your preg go past your due date the ideal time to deliver according to a recent very large study is 39-40 weeks , I do not let any of my pts go past 40 weeks !