35 y/o female presents with gradual onset of ascending bilateral lower extremity numbness over the past several days. Neuro exam otherwise unremarkable with exception of said numbness up to midabdomen. Could this be Guillain-Barre syndrome (GBS)?

Unlikely.

Features that make GBS doubtful:

• Severe sensory signs with limited weakness at onset 
• Sharp sensory level 
• Bladder or bowel dysfunction at onset 

Classically GBS presents with progressive weakness in both arms and legs associated with areflexia.  Other features which strongly support the diagnosis include:

• Progression of symptoms over days to 4 weeks 
• Mild sensory symptoms or signs 
• Autonomic dysfunction 
• Pain
• High concentration of protein in CSF 
• Cranial nerve involvement, especially bilateral weakness of facial muscles 
• Relative symmetry of symptoms

Source

van Doorn, P et al.  "Clinical features, pathogenesis and treatment of Guillain-Barre syndrome"  Lancet Neurol 2008.