Visuddhimagga XVI-40

 Yaṃ vijāyamānāya mātuyā kammajehi vātehi parivattetvā narakapapātaṃ viya atibhayānakaṃ yonimaggaṃ paṭipātiyamānassa paramasambādhena yonimukhena tāḷacchiggaḷena viya nikkaḍḍhiyamānassa mahānāgassa narakasattassa viya ca saṅghātapabbatehi vicuṇṇiyamānassa dukkhaṃ uppajjati, idaṃ vijāyanamūlakaṃ dukkhaṃ.

（四）在母亲生产之时，胎儿受苦，由于业生之风倒转，如堕地狱，然后向于可怖的产道，从极狭窄的产门而出，如从键孔拉出大龙，或如地狱有情为两山研成粉碎相似。这是由于分娩之苦。

Ñ(XVI,40): The pain that arises in him when the mother gives birth, through his being turned upside-down by the kamma-produced winds [forces] and flung into that most fearful passage from the womb, like an infernal chasm, and lugged out through the extremely narrow mouth of the womb, like an elephant through a keyhole, like a denizen of hell being pounded to pulp by colliding rocks—this is the suffering rooted in parturition.