In a form, should full sentences or just "labels" be used to specify what information should go in each section?

It would seem to be common for labels to be used beside input fields instead of asking an explicit question, to convey what information the user should fill out a form with. Since this is common practice, is it a good thing though it may be unnatural?

For example when filling out a form:

Country of birth: ___________
Birthdate: ___________
Last employer: _________

vs

Which country were you born in? _____________
What is your birthdate? ___________
Who was your last employer? ___________

vs

Specify the country you were born in: _____________
Specify your birthdate: ___________
Specify your last employer: ___________

In forms, is it better not to phrase it as a question even though in ever day speech you would ask the question if you wanted to know the information?