Before dying in the fall, a single weed can drop thousands of seeds that will be ready to germinate in the spring.
Crabgrass is a problem, but you can take care of it now by applying a lawn food that contains a pre-emergent such as Scott's Turf Builder with Halts Crabgrass Preventer. Make sure to apply before temperatures reach 65 degrees.
It will feed your grass while effectively providing a barrier that prevents crabgrass seeds from germinating. STOP the problem before it starts!