The Compendium of Bheḷa 4.4.32

4.4.32

tatrāha kathaṃ garbho māturudare tiṣṭhatīti | ūrdhvam iti śaunakaḥ | avākchirā iti bharadvājaḥ | nety āha bhagavānpunarvasurātreyaḥ | yady ūrdhvaṃ tiṣṭhet tarhi mātṛghātī syāt | yady avākchirāḥ svaghātī syāt | kathaṃ tarhi? tiryak sarvairayamaṅgapratyaṅgaiḥ pratibhugnaḥ śete ||tasya tadananataraṃ tatprathamaṃ pratipadyate [sandarbheṇānena pratipipādayiṣito ' rthaḥ na spaṣṭaḥ] | tasmāt tasya śiraḥ prathamaṃ punarvasurātreyaḥ pratipadyate | tadasya gurutaraṃ bhavatīti | atha khalu vṛkko medo gururiti saṃpravṛddhau parasparamabhivardhayanti ||

On this he asked: How does the fetus stay in the mother's belly? Śaunaka said: Upright. Bharadvāja said: Head downwards. The venerable Punarvasu Ātreya said: No. If it stood upright, it would be a slayer of its mother; if it were head downwards, it would be a slayer of itself. How then? Transversely, bent in on itself with all its major and minor limbs, it lies. […] Therefore, Punarvasu Ātreya maintains that its head [develops] first, because it is heavier for it. Now indeed, when the kidney and fat are grown heavy, they mutually enlarge one another in their development.