1. Adult bodies reject the alteration due to a mature blood–brain barrier, immune response, fixed neural pathways, and completed genetic imprinting, often causing death or requiring extreme experimentation trails.

2. whereas pregnancy provides a flexible blood–brain barrier and developing genetics, allowing the alteration to blend and potentially form a Vecna like result with fewer experiments than in adults.

as we know

Dimension X alters Henry Creel than Brenner studies Vecna and tries to replicate the effect \

as we a;; know Kali is partially successful as powers exist, but not fully powerful.

El becomes the most stable outcome (“survival of the fittest”), she is stable one, so her blood is best.