The spice agony ritual involves a Bene Gesserit acolyte drinking the Water of Life, a poison, which she must internally convert to a harmless substance. She who survives this ordeal becomes a Reverend Mother; the agony awakens Other Memory and the Reverend Mother can access the full memories of all her female ancestors. If, however, the Reverend Mother is pregnant at the time of drinking the Water of Life, the same will happen to the fetus or fetuses she carries.
It is with reason and terrible experience that we call the pre-born Abomination. For who knows what lost and damned persona out of our evil past may take over the living flesh?
Hence the danger of Abomination: Reverend Mothers can resist the inner voices because as adults they have a full personality and a very solid sense of self. The pre-born don't, and so they become very vulnerable to the tides of ancestors.
Interestingly, it is also hinted in Children of Dune that the word Abomination refers also to Bene Gesserit who abuse their metabolic control in order to extend their lives. The Sisterhood feels that public knowledge of the Bene Gesserit's capabilities for near-immortality would result in a devastating backlash against them:
These words spoken so easily touched on a subject the Bene Gesserits made almost unthinkable. Many Reverend Mothers could choose that course ... or try it. The manipulation of internal chemistry was available to initiates of the Sisterhood. But if one did it, sooner or later all would try it. There could be no concealing such an accumulation of ageless women. They knew for a certainty that this course would lead them to destruction. Short-lived humanity would turn upon them. No — it was unthinkable.
In Dune Messiah, the twin children of Paul Atreides, Leto and Ghanima, also become pre-born as a result of high levels of spice in the diet of their mother, Chani, during their gestation. Later in Children of Dune, Leto and Ghanima avoid becoming "Abominations" through the protection of benign ancestral memory-selves. Ghanima actually finds the cure for the Abomination syndrome while trying to conceal truth through a form of deep hypnosis.
In another quote from the Azhar Book, the conditions for Abomination are stated: "First a pregnant Bene Gesserit breeder must ingest the Water of Life ... This chemical, carried in her blood to the womb, activates the fetal psychic awareness and produces a babble of sound and sensory imagery which the un-born is unable to comprehend or assimilate. At birth, this "awakened" baby supposedly sees with adult comprehension because of the active, intelligent, adult memories it now carries at a conscious level. The child, therefore, appears to the uninitiated as extremely precocious but to the knowledgeable as a possible Abomination."