I count like Clysta does. At 20 weeks you are 5 months so at 22 weeks you are 5 1/2 months and at 24 weeks you are 6 months. I like counting by weeks, it's just easier.
I have always counted 5 months to be 20 weeks. It depends on how you look at it. You can go by calendar days, or 4 week intervals. Going by 4 week intervals though, you're also pregnant for 10 months though, not 9. This is just because you have to account for the extra days in a month, since only one month is 28 days.