Ask your vet about fescue in the area, but don't worry as mares can deliver with little or no udder, and be full within hours of foaling. Or you're concerned, you can let your vet know you may need momma to be given a shot if she's dry after delivery, but usually all is well.
With baby riding so wide, she has time to build that udder in the next few weeks, so I believe all will be well.