I am by no means an expert but since we have a 3 week old filly I will tell you how it went for us. What you are seeing is called bagging up. You will notice the bags above the teats and generally that whole area under there will start to swell as the mare gets closer to partruition, ie foaling. This is all normal as the mares body is getting ready for foaling. As she gets closer the bags will get bigger and fuller, sometimes unevenly at first. What you want to watch for is the teats to change the direction they are pointing. What I mean is the closer she gets to foaling the more they will point straight down. At first they will point inward. You'll know what I mean when you see it. Our mare went about 5 days over her 'due' date. Yours being a 'maiden' mare, it is not uncommon for them to go to almost a full 360 days. I hope this helps. Generally, 99% of the time, nature does everything just right. So sit back and enjoy and try not to stress out as much as I did... Good luck!