The Compendium of Caraka 4.8.17

4.8.17

yathoktena vidhinopasaṃskṛtaśarīrayoḥ strīpuruṣayormiśrībhāvamāpannayoḥ śukraṃ śoṇitena saha saṃyogaṃ sametyāvyāpannamavyāpannena yonāvanupahatāyāmapraduṣṭe garbhāśaye garbhamabhinirvartayatyekāntena/

When a woman and a man, whose bodies have been prepared in accordance with the prescribed procedure, attain union, the unvitiated semen, having united with the unvitiated ovum in an uninjured genital tract and an unvitiated uterus, invariably produces an embryo.