The second trimester traditional stands as weeks 13-24.............4th month starts at 16 weeks........... 5th month starts at 20 weeks

Each month is not 4 weeks.

This is why pregnancy is counted in weeks.

If you really want to count months then count calendar months.

So if you last period was on the 15th of june then on the 15th of july you are one month, on the 15th august you are 2 months and so on.

Otherwise just count weeks