The Collaborative International Dictionary of English v.0.48:

Birthmark \Birth"mark`\, n.
   Some peculiar mark or blemish on the body at birth.
   [1913 Webster]

         Most part of this noble lineage carried upon their body
         for a natural birthmark, . . . a snake.  --Sir T.
                                                  North.
   [1913 Webster]

WordNet (r) 3.0 (2006):

birthmark
    n 1: a blemish on the skin that is formed before birth [syn:
         birthmark, nevus]

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "birthmark":
   blackhead, blaze, bleb, blemish, blister, blotch, brand, bulla,
   caste mark, character, check, checkmark, cicatrix, comedo, crack,
   crater, craze, cut, dapple, defacement, defect, deformation,
   deformity, discoloration, disfiguration, disfigurement, distortion,
   dot, earmark, engraving, fault, feature, flaw, fleck, flick,
   freckle, gash, graving, hack, hemangioma, hickey, jot, keloid,
   kink, lentigo, macula, mark, marking, milium, mole, mottle,
   needle scar, nevus, nick, notch, patch, pimple, pit, pock,
   pockmark, point, polka dot, port-wine mark, port-wine stain, prick,
   puncture, pustule, rift, scab, scar, scarification, score, scotch,
   scratch, scratching, sebaceous cyst, speck, speckle, splash, split,
   splotch, spot, stain, stigma, strawberry mark, sty, tattoo,
   tattoo mark, tick, tittle, track, trait, travail, twist, verruca,
   vesicle, wale, warp, wart, watermark, weal, welt, wen, whitehead