The Compendium of Caraka 4.6.24

4.6.24

sa copasthitakāle janmani prasūtimārutayogāt parivṛttyāvākśirā niṣkrāmatyapatyapathena, eṣā prakṛtiḥ, vikṛtiḥ punarato 'anyathā/

And when the time of birth has arrived, by the action of the labor-wind, turning around with head downwards, it emerges through the genital tract; this is the normal condition, but what is otherwise is abnormal.