Devaki conceived Krsna by divine conception as the child was transferred into her womb by mind to mind transference. Krishna was the 8th child of Devaki and also the 8th avatar of Vishnu.
From Śrīmad-Bhāgavatam
“Thereafter, accompanied by plenary expansions, the fully opulent Supreme Personality of Godhead, who is all-auspicious for the entire universe, was transferred from the mind of Vasudeva to the mind of Devakī. Devakī, having thus been initiated by Vasudeva, became beautiful by carrying Lord Kṛṣṇa, the original consciousness for everyone, the cause of all causes, within the core of her heart, just as the east becomes beautiful by carrying the rising moon.
Source: Śrīmad-Bhāgavatam (Bhāgavata Purāṇa) 10.2.18
“Devakī then kept within herself the Supreme Personality of Godhead, the cause of all causes, the foundation of the entire cosmos, but because she was under arrest in the house of Kaṁsa, she was like the flames of a fire covered by the walls of a pot, or like a person who has knowledge but cannot distribute it to the world for the benefit of human society. v19
“Because the Supreme Personality of Godhead was within her womb, Devakī illuminated the entire atmosphere in the place where she was confined. Seeing her jubilant, pure and smiling, Kaṁsa thought, “The Supreme Personality of Godhead, Viṣṇu, who is now within her, will kill me. Devakī has never before looked so brilliant and jubilant.” v20
