Doctors use growth charts to track a child's physical growth over time. At each checkup, the doctor or nurse will measure your child's weight, height, and head size, and then compare it to a chart that shows the average for babies that are your child's age and sex. The doctor will then tell you how your child compares to other kids in the same developmental stage using percentiles.
Don't worry too much about the percentiles. Every child is different and will grow at his or her own pace. The most important thing is that your child's growth curve continues to go up or stabilize — and not drop too much.