I have looked into this before. While I cannot definitively say that President Abraham Lincoln had no daughters, stillborn or otherwise, I can tell you that I have never found documentable or creditable evidence of any female offspring. There are many examples of "Theory as fact" regarding a Lincoln daughter that have only conjecture to back them up; and not a few conspiracy theories out there as well. And then, of course, there is the book "Lincoln's Daughter" which is a work of fiction.
I think this may be a wild goose chase, Brian, but I'd sure like to know if you find out otherwise!