She looks like she is a week or two away. Or less, depending on the mare. Her tail/hip area is slacked off and her udder is pretty full, just looks like she will fill up her teats a bit more.
Is there somewhere else where she can foal? Somewhere more sheltered? All that mud and the very wide spaced white fencing won't be ideal for a newborn. I agree on getting her fatter. Any mare and foal grain pellets will come with supplements in it (or it should, check the back of the bag) then you just need to make sure she has free choice salt minerals and lots of alfalfa type hay. Good luck!!
A good horse is worth more than riches. ~ Spanish Proverb