The name given for the garment style 'baby doll' was popularized by the movie of the same name in the late 50's which portrayed the title female character as a young and sexy nymphet and the whole idea of that set off the garments popularity amongst young women.
Both Twiggy and Goldie Hawn were most associated with the baby doll style of the 60's...
The style faded a bit through the late 70's and 80's but made a resurgence in the 90's when grunge goddess Courtney Love gave it a new spin and the birth of the punk rock style called 'kinderwhore' was born! Basically it's a baby doll dress that has been torn, ripped up or cut low in the front paired with platform Mary Jane's.
The trend had died down again a bit but popped up in the 'here and now' as designer Julien MacDonald fashioned up quite a variety of baby doll dresses circa 2011 as he showed his collection at the Spring Fashion week.
If you notice, there hasn't been too much evolution regarding the shape of the style with the baby doll dress, just a lot more variety where color and textures are concerned.
pics: eliakazanmovies.blogspot.com, corbisimages.com, hubpages.com, fashiontheory.umwblogs.org, threndsandtreats.wordpress.com