Barrayar by Lois McMaster Bujold

Though this novel was actually published a number of years after The Warrior's Apprentice, where we meet Miles Vorkosigan for the first time, it precedes that book in the Vorkosiverse timeline. It picks up shortly after the end of Shards of Honor. Cordelia and Aral are married and living on Barrayar, expecting their first child. Emperor Ezar is dying, and needs Aral to act as Regent for the five year old boy, Gregor, until he reaches his majority. Aral definitely does not want the job, but Ezar points out to him the serious flaws in all of the other candidates for the position, and Aral is forced to do the task.

His appointment, followed shortly by the death of the Emperor, triggers plotting by those who wanted the power for themselves. One of those consequences is the soltoxin grenade attack on Aral's home, which leaves him and Cordelia briefly poisoned, while the antidote to the poison threatens to destroy the skeletal structure of the child in her womb. They take the drastic measure of placing their child in one of the recently acquired uterine replicators (which Aral fortuitously took charge of at the end of Shards), bombarding the baby with massive amounts of bone-enhancing therapies while he is growing.

When one of Aral's colleagues in the council of counts decides he must have the regency for himself, he takes Princess Kareen, Gregor's mother, hostage, claims that Aral has had the boy murdered, and begins to imprison and execute any of Aral's loyalists he can find. Ezar's old head of Security, Captain Negri, escapes the fighting with the boy emperor and in his last dying effort, delivers him to Aral for safekeeping. Aral's father and Cordelia and Sargent Bothari take the boy and escape into the mountains to hide, leading Vordarian's troops on a wild goose chase while they attempt to capture him.

When the man charged with fetus Miles' care shows up suddenly to tell Cordelia and Aral that Vordarian has seized the replicator and that it is being held hostage in the palace, Cordelia defies Aral and heads into the capitol to rescue her son.

This book is filled with ties to tons of the other events in the rest of the Vorkosigan series, and provides a number of in jokes later on for dedicated readers, as well as filling in some of the gaps in characters' backgrounds. Another one of the books that has grown on me with time.