There several stories regarding the birth of Kamsa (Kansa), the maternal uncle of Sri Krishna. The popular belief is that Kamsa is the son of Queen Padmavati and King Ugrasena of Mathura. But as per Bhagavad Purana and other several other Puranas King Ugrasena did not father Kamsa.
Kamsa was born to Dramila, a Gandharva, and Ugrasena's wife Padmavati.
Padmavati who was in her monthly periods walked in the forest. Dramila saw her and felt attracted to her. He violated her and made her pregnant.
Angered at this, she cursed Dramila thus: "Since you have procreated a child in me, while knowing that I am wife of another man this son will be killed by a boy born in my husband's family."
Years later Kamsa ascended the throne after putting King Ugrasena in prison.
According to the curse of Padmavati, Sri Krishna was born in Ugrasena's family and he killed Kamsa.