She doesn't look all that close to me. Her belly isn't dropped (although some mares never do quite drop), and her udder is nowhere near ready (although some mares don't bag up until delivery), and her rear end doesn't look all that soft/sunken (although that never happens with some mares).
I'm not sure a vet would be able to tell you much more, either - when we were going through this the vet just said "all you can do is wait and see!". Unless they palpate and find the foal's head right there and ready to go.
I will say though that if she was bred in December she could go any day now, or she could wait another few months. Make sure that you have frozen colostrum on hand, just in case she doesn't bag up before foaling.