I am almost 29 weeks when will they do another ultrasound to see how big the baby is?

I mean measuring correctly doesnt tell me how big the baby is. I am low-risk so only get seen once a month and this appointment is the first thats in 3 weeks. when will my OB tell me how big the baby is? I have an appointment this coming Monday. I willa sk her then but in the meantime want 2 know.