By weeks, at 20 weeks you are 5 months. That's how I count it. Every 4 weeks is a month.
I always considered 5 months to be 20 weeks personally. It depends on how you look at the charts and calendars as well, because that follows a 4 week month, and most months are not 28 days, but rather 30-31. Accounting for the extra days you're actually pregnant for 10 months, not 9.
im glad you asked...im curious too :)