Unless she was a premie.

I was thinking the same thing.

It's also possible the parents were guessing her weight since they used a round number of 10 lbs. Also they may have used a last weigh-in from a doctor's visit.

10lbs at 3 months old can be normal. especially if born premature or just not a chunky kid.

My daughter was that at 3mo. and she was 7'10 at birth. She had malabsorubtion problems and anemic. This contributed to her not gaining weight. It took over a year to finally get her system straighten out. She doing fine today.