I've had growth scans with all three of mine even though they were all low risk. First one, baby was measuring on track then started measuring a week behind, then two weeks behind so at about 34 weeks I went for a growth scan. She was fine, just small! 2nd baby I had a different midwife at to usual at one appointment (early 3rd trimester) and she measured me three weeks ahead
so she sent me for a growth scan. Again, all was fine, she just measured me differently to how my normal midwife measured me. This baby has measured 2 weeks ahead all along. Normal midwife not concerned, but I went in to the hospital for reduced movements at 31 weeks and the midwife there was concerned so sent me for a growth scan again. In my experience they seem to be pretty vigilant and will send you for one quite readily if they think you need it x