Growth scan?

Has anyones doctor said anything to them about having a growth scan done? I am almost 30 weeks pregnant and I have been reading about other Moms having these done in the third trimester at around 36 weeks but my doctor hasn't said anything to me about this. I am going to ask her at my next appt but I was just curious if anyone else knows if this happens with every pregnancy or only if their is a problem? Thanks girls!!