Is dr ordering a growth ultrasound normal?

My dr noted yesterday that heartbeat sounded good but my baby is measuring a week ahead. She originally said no ultrasound until 29 weeks(I'm 21 weeks and had my last ultrasound at 16 wks) but as I was leaving yesterday she changed her mind and said she wanted a "growth" ultrasound done in 4 weeks. She went into another room before I could ask if everything is ok. Should I be concerned or are these growth ultrasounds a normal practice?