A phenomenon that affects play very rarely. As a result, they are not discussed in elementary textbooks and even highly advanced discourses usually mention them only briefly. They are stations that were planned but never actually built, for example the Northern Line extension from Edgware to Bushey Heath, or the originally intended continuation of the Jubilee Line from Charing Cross to Docklands via Aldwych. The tunnels for this were actually built as far as Aldwych (which makes Aldwych a true foetal-ghost for the Jubilee, as well as now being a standard ghost on the Piccadilly.)
The most celebrated Foetal Ghost station is North End, otherwise known as Bull & Bush; this station was actually partly built at track level but never opened to the public - nor linked to the surface. Had it been fully built it would have been the deepest station on the entire network. This remarkable non-station has had many interesting uses over the years, however, and indeed requires a special rule all to itself, being treated differently from the few other foetal ghosts.