concerned with head growth

My LO needs was born at 31 weeks exactly. She has been doing really well growth wise on the preemie chart. However, we just had her 4 month (actual) appointment and her head circumference is only in the 6th percentile. At this point her pediatrician doesn't seem too concerned and is going to keep monthly measurements when we go in for her synagis shots. Was anyone else's preemies head on the smaller side? Just looking for comfort and reassurance 😊