In medias res does not mean that a story is narrated from its middle. It means that a story begins in the middle of events instead of introducing the characters and the setting first (this is called ab ovo, "from the egg onwards").
Stephenie Meyer's Twilight does not employ in medias res: the novel opens with a slow and quiet scene in which the protagonist moves to a new home; we learn of her family background, and her new surroundings are extensively described. The events that are relevant to the story in Twilight begin after this introduction and not "in the middle of things".
Horace coined the term in medias res when he praised Homer's Iliad in his Ars Poetica for not beginning his narration of the Trojan War at its beginning but in the middle of things. The Iliad begins at a point in the story where Troja is already besieged by the Greeks and Chryseis is held captive by Agamemnon. The narration does not start in the middle (where Hector bursts through the defensive wall) or near the climax (when Achilles stabs Hector), but it does start in the middle of an existing problem.
The following schema illustrates the difference between narration that begins in the middle of a story, and a story that begins in medias res:
You can think of a story that begins in medias res as one story in a sequence of stories. For example, the Iliad is preceded by the works of Euripides, Sophocles, and other literary works and legends about the Trojan War. Twilight on the other hand is the first story told in its world and it does not have a prehistory that its protagonist is already involved in at the beginning of the narration. Bella's story begins as she moves to Forks and meets Edward there.
Stories that begin in the middle, usually tell their beginning "in flashback". A flashback is "an interjected scene that takes the narrative back in time from the current point in the story" (from Wikipedia). Sometimes a large part of the story or all of its beginning is told in flashback from its middle, as in the Odyssey:
In the Odyssey, most of the adventures that befell Odysseus on his journey home from Troy are told in flashback by Odysseus when he is at the court of the Phaeacians. The use of flashback enables the author to start the story from a point of high interest and to avoid the monotony of chronological exposition. (Encyclopædia Britannica, s.v. Flashback)
In my schema above, the dotted part of the story that begins in the middle is told in flashback. See also the illustration below.
Apart from stories that begin in medias res and stories that begin in the middle, there are also stories that prepend an episode from the middle to their beginning. These stories employ a
A flashforward is "a scene that temporarily takes the narrative forward in time from the current point of the story" (from Wikipedia). Often, especially in crime fiction, a flashforward is used as a prologue.
The following illustration shows a narrative beginning in the middle of the story and recounting the beginning of the story in flashback (left) as well as a flashforward to the middle of the story prepended to the narrative (right):
I believe "flashforward" is the term you are looking for. There is no special term for narratives that begin with a flashforward.
There is a term for a story told in reverse order from the end to the beginning, and that is "reverse chronology", but that is not what you're asking.