Stretch marks are usually referred to in the plural (striae).
Agree with Benson and Tweety that although there are clearly other medical causes, "stretch marks" most commonly conjure up an association with pregnancy.
Dorland's Medical Dictionary:
striae atrophicae: linear, depressed, atrophic, pinkish or purplish, scarlike lesions that later become white (striae albicantes, lineae albicantes), occurring on the abdomen, breasts, buttocks, and thighs. They are due to weakening of the elastic tissues, and are associated with pregnancy (striae gravidarum), excessive obesity, rapid growth during puberty and adolescence, Cushing's syndrome, or topical or prolonged treatment with corticosteroids. Called also striae distensae, lineae atrophicae, and linear atrophy.http://www.mercksource.com/pp/us/cns/cns_hl_d...
Thieme Leximed Medizinishes Wörtebuch:
"Schwangerschaftsstreifen pl gyn. stretch marks"
E: striae; stretch marks
so the current LEO entry should remain as it is or be changed to the plural - either way it needs to be kept in LEO.
Neither Thieme nor Roche has an entry for "Dehnungsstreifen" but in my trusty old Wörterbuch der Medizin und Pharmazeutik (Bunjes, 1981):
"Dehnungsstreifen m pl (Haut) atrophia striata"
so an entry for Dehnungsstreifen / stretch marks could be added.
Another suggestion would be to add the Latin terms in explanation.