Miles Vorkosigan is the son of Aral Vorkosigan of the planet Barrayar and his Betan wife Cordelia Naismith. He has several physical impairments due to an assassination attempt directed at his parents. A soltoxin gas grenade was thrown into their bedroom during Cordelia's pregnancy with Miles but the couple's lives were saved by quick administration of the antidote—a violent teratogen that destroys skeletal development in a growing fetus.

The only available treatment for the fetus was experimental at best, and would have been fatal to the mother. Miles was therefore transferred to an artificial uterine replicator in an effort to salvage his bone development. While in the replicator, Miles was kidnapped from the hospital and used as a hostage against his father by Vidal Vordarian, the leader of the Vordarian pretendership, putting a temporary halt to the treatments. He was ultimately born with "chalk-stick bones, friable as talc", and is described as "wizened as an infant homunculus.