*READ DESCRIPTION* should i be worried my 6 month old doesn’t laugh?

My 6 month old was 4 weeks premature, so I understand she could be behind a little bit, but with almost everything else she has been right on track. She smiles, shrieks, screams, gets excited, but no laughing. We have tried EVERYTHING to the point we look like clowns. Should I be worried? Going to talk to her ped about it next week at appt.

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