Babies can just be very serious, right?

My niece is 9 months old and rarely smiles. She didn't start smiling until she was three months old and even now it's very, very seldom. She'll flash a smile occasionally and when she was a younger infant she would smile at the ceiling fan (all babies love ceiling fans) but that's pretty much it. When I try to play peek-a-boo with her she just stares at me blankly but does make eye contact. She always has the same expression. My daughter is the same age just a couple weeks younger and in all of their pictures she has the same open mouth expression in every single photo. I went through my BIL social media photos last night and same thing, she has the same face in every photo.She is extremely cute and IS hitting all of her gross motor milestones she seems inquisitive but when other babies laugh she stares like she doesn't get the joke. She's very laid back and not at all fussy. She just never smiles. My husband is an MD and feels something is off but doesn't know enough about pediatrics to say anything other than "Lilly is very serious."But are we just making a comparison between our children and the other children we know? Is it not my place to worry because with time, if there was a problem would become apparent to everyone? Can not smiling or interacting much be a little unusual but totally normal so not at all cause for concern.Thanks! via /r/Parenting http://ift.tt/2rIWBbV