What does it mean for a baby to cross percentile lines?

Not much: Percentiles compare us to others at the same age. If we are in the 80th height percentile, then we are taller than 80 out of 100 kids the same age. We grow at different levels sometimes though, and we can be 80th percentile one month and then 30th percentile 3 months later. As long as kids have a progressive gain, they are probably fine. Acute changes should be addressed by your doc.

See below: The percentile lines are an estimate of the growth of a child, and help the pediatrician make sure your child is growing correctly. If the child starts crossing the lines, the doctor will be making sure nothing is going wrong. This could include weight loss, or change in head size, which might indicate that something serious is going on. Talk to your pediatrician about your concerns.