Is this a major problem to worry about ?

ok i drive a old second hand car holden barina hatch 1991 year sometimes when i start up my car i get white smoke comming out of my exhaust is this normal ? i been told it could be the head gasket thats on its way out but the only thing is once i drive the car it dosnt smoke anymore it only smokes when i start it then its fine. i even told my mechanic that i get white smoke here and there he even said so him self it wont be your head gasket if it only does it on start up its probly something els maby like my car needs a service i dont know whats going on as im trying to get opnions any ideas ??? also my car is cabirator ?