How can I tell how many weeks a rat is pregnant?

So I've been volunteering at this animals shelter and I love rats so I always pick them up. There are 2 rats, one of them is fine and always comes out but the other one is a bit shy. I picked up the shy one and noticed that her stomach was rounded at the sides, and it doesn't feel like fat cause it's hard, I'm pretty sure she's pregnant. I think it's about 3 weeks cause that's usually when they start to show. Do any rat owners know how far off she is from what I told you?