Mahāpadānasutta

Suttapiṭaka · Dīghanikāya · Mahāvagga · Vol. 10 · Section 21

[21] Bhikkhus, this is the rule of nature (dhammatā): whereas other women give birth sitting or lying down, the mother of the Bodhisatta (bodhisatta) does not give birth in that way; the Bodhisatta's mother gives birth to the Bodhisatta only while standing. This is the rule of nature in this case.

English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.