When is it legislated to perform the 'aqeeqah for a newborn in the seventh month, while they are still in the hospital until they complete seven months?

The Sunnah is for the 'Aqiqah to be on the seventh day of birth. If that is missed, then on the fourteenth. If that is missed, then on the twenty-first. If that is missed, then at any time it becomes feasible. There is no difference in this regard between one whose gestation period was short and one whose usual gestation period was complete.