Ragnar had returned from his third raid of Engaland and wanted to shag Aslaug as soon as he got home. Aslaug, who was something of a soothsayer, has a foretelling and tells Ragnar that if he takes her she would give birth to a monster. Being a Goteland princess and the daughter of a famous dragonslaying Viking, he cant resist.
Ivar was concieved and born with a lower limb deformity. As such Ragnar must abandon the child to die in a forest. This he does, even though he loves all his children. It was Aslaug who followed Ragnar into the woods, retrieved the newborn, and spared his life.
As one of the sons of Ragnar, Ivar the Boneless would indeed become one of the cruelest norseman in history.
This is taken from season 2 of History Channels Viking series, not really academic proof but makes great television drama.