Does the seventh month of pregnancy start at 27 or 28 weeks?

About 28 weeks. The easiest way to figure out how many months you are is to count backwards from your due date which is of course 9 completed months. 4 weeks is a little less than one month and so 40 weeks = 9 months.
3rd trimester: >28w. As most months are 4-5 weeks long, the seventh month starts at 28 weeks.