WordNet (r) 3.0 (2006):

uncontrived
    adj 1: not by design or artifice; unforced and impromptu; "an
           air of unstudied spontaneous utterance is apt to be
           painstakingly achieved"; "simple unstudied charm" [syn:
           unstudied, uncontrived] [ant: studied]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "uncontrived":
   ad-lib, caught napping, caught off balance, caught short,
   extemporaneous, extemporized, haphazard, hasty, impromptu,
   improvised, makeshift, precipitate, rough-and-ready, snap,
   surprised, taken aback, taken by surprise, taken unawares,
   tripped up, unarranged, unbegun, unconcocted, undeliberated,
   undevised, unhatched, unmade, unmanufactured, unorganized,
   unplanned, unpremeditated, unprepared, unprimed, unready,
   unstudied