If you're born without a sift spot or yur it closes prematurely, does that automatically mean you have craniosynostosis?

Maybe: Craniostenosis is premature suture fusion diagnosed by examination, x-ray and ct. The soft spot can be hard to feel. If it is felt that there is asymmetry is the head shape this can mean craniostenosis. So the answer is maybe- see your pediatrician.

Craniosynostosis: No is the answer - children can have the soft spot close early and it can mean absolutely nothing. The key question is whether the brain and skull are still growing and the child is making normal milestones - if that is the case you can ignore the premature closure.