17 wk old niece head circumference is on 99th percentile.50th percentile for height and weight. is this a problem? her mother has quite a large head.

No: this is not a problem. Head size is often in a different percentile than other body parts. My daughters baby is currently doing the same thing as did her baby before that.

Probably fine: It's fine for an infant to have a large head, as long as it stays at a fairly constant percentile. It's only when heads grow or shrink too quickly that the child should be evaluated further. Make sure to talk with ur baby's pediatrician if u r concerned.

Maybe: There is a condition of familial macrocephaly (big head). Her pediatreician should be keeping a head circumference chart. The growth rate recorded on the chart is an indication of health or concern.