Tradition and history — not medical advice
What the Yoga Sūtra and Haṭha Yoga Pradīpikā say about posture — and about pregnancy
"Āsana" (posture) gets a one-line definition in the Yoga Sūtra and a chapter of named postures in the Haṭha Yoga Pradīpikā. Neither text says a word about pregnancy.
What the texts say
Yoga Sūtra of Patañjali — Sādhana-pāda 2.46–2.48
The posture should be steady and comfortable. In effortless relaxation, dwell mentally on the Endless with utter attention. From that there is no disturbance from the dualities.
BonGiovanni, The Yoga Sutras of Patanjali: The Threads of Union. Public domain.
Haṭha Yoga Pradīpikā — Chapter 1, verse 34
uttānaṃ śabavadbhūmau śayanaṃ tachchavāsanam | śavāsanaṃ śrānti-haraṃ chitta-viśrānti-kārakam || 34 ||
Lying down on the ground, like a corpse, is called Śava-âsana. It removes fatigue and gives rest to the mind.
Pancham Sinh, The Hatha Yoga Pradipika (1914). Public domain.
Neither text mentions pregnancy anywhere: not once across the 194 verses of this edition of the Yoga Sūtra, and not once across the Haṭha Yoga Pradīpikā's four chapters. This page's own search of both public-domain translations found no verse addressing a pregnant practitioner, a modified posture for pregnancy, or any pregnancy-related caution.
What modern guidance says
Keep up your normal daily physical activity or exercise (sport, running, yoga, dancing, or even walking to the shops and back)
NHS (reviewed 2023-03-15)
do not lie flat on your back for long periods, particularly after 16 weeks, because the weight of your bump presses on the main blood vessel bringing blood back to your heart and this can make you feel faint
NHS (reviewed 2023-03-15)
ACOG's exercise-in-pregnancy guidance discusses how pregnancy hormones loosen joint ligaments, and advises against jerky, bouncy or high-impact movements — see ACOG's own FAQ for its full advice.
Speak to your doctor or midwife
A prenatal yoga class run by a qualified instructor, and a conversation with your doctor or midwife about your own pregnancy, are the two things worth having before starting or changing any exercise routine — not a classical text that was never written with a pregnant body in mind.