Nearly 9 weeks and not smiling

Hannah

My son was born on the 19th October 2018 at 35+4 weeks. He has not yet smiled genuinely (not because of wind), they say between 6-8 weeks old is when babies start to smile and if they haven’t done so yet it’s nothing to worry about. I’m wondering if it’s because he was preterm? And if I should actually be tracking his development on his actual due date rather than from when he was born?