Various limits apply to most tax deductions, and one type of limit is a “floor,” which means expenses are deductible only if they exceed that floor (typically a specific percentage of your income). One example is the medical expense deduction.
Because it can be difficult to exceed the floor, a common strategy is to “bunch” deductible medical expenses into a particular year where possible. If tax reform legislation is signed into law, it might be especially beneficial to bunch deductible medical expenses into 2017.