In the Dune saga (1st three books) why aren't Fremen children born with prescience or pre-born? I get that Chani was put on a high spice diet when pregnant but surely given that all adult Fremen have the blue eyes and participate in the Spice orgy their systems would be so laced with Melange that it would transfer to their unborn children?
Becoming a Reverend Mother requires the Bene Gesserit sister to take and change the bile of a dying sandworm into the Water of Life using her prana-bindu training1 (literally creating a molecular catalyst out of individual molecules). During that process, if the mother is pregnant, her child may also gain the ability to access those same genetic memories but without the complex Bene Gesserit mental training that allows them to selectively control those memories.
In the absence of a revelatory moment (the moment in which a neophyte Bene Gesserit become a full Reverend Mother), merely taking large amounts of Spice isn't sufficient to awaken genetic memory in the children of any pregnant Fremen women2.
The Dune Encyclopedia makes the steps required abundantly clear
The Azhar Book outlines the conditions necessary to a state of possession (Mahrana IV: H-K). First a pregnant Bene Gesserit breeder must ingest the Water of Life, apparently an hallucinogenic chemical which varied during the history of the order. 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 com- prehension 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. Only bright Bene Gesserit children were suspect; precocious lay children were safe from scrutiny.
1Jessica mentions that the process of becoming a Reverend Mother at the Chapter House uses a different method (albeit one that also uses spice) but is basically the same in outcome
2Note that Leto and Ghanima are also pre-born, but through their genetic inheritance from Paul and hence can be discounted from this discussion.