Mahapadanasutta 1.24.1

1.24.1

Dhammatā esā, bhikkhave, yathā aññā itthikā nisinnā vā nipannā vā vijāyanti, na hevaṃ bodhisattaṃ bodhisattamātā vijāyati. Ṭhitāva bodhisattaṃ bodhisattamātā vijāyati.

It’s normal that, while other women give birth while sitting or lying down, not so the mother of the being intent on awakening. She only gives birth standing up.