Anyone else confused on the months they are?

A❤️C

I'm 21 weeks. When I was like 19 weeks my sister told me I'm like 5 in a half months. I thought 5 months was 21 weeks? Or is it more like 6 months?

I've seen the picture things people post on here but I'm still confused. Idk what to tell people when they ask how many months I am. Lol