Pregnancy weeks and months

paige

So I see some woman posting on here how far along they are on months one lady said im 3 months exactly today and some one commented and said how many weeks and she said 12 exactly well according to the photo that would be 3 1/2 months right? And then today I seen a girl say she was exactly 5 months today so I commented and said how many weeks and she said 20 but the photo says 5 1/2 I just need some clarification here as to who is right and if a lot of woman are misunderstanding how months actaully work?