Sorry to say but from the picture it is impossable to tell. I have several laid off from work brood mares that look just like that because they are eating too much hay, can we say fat? did the previous owner say she was bred, and if so when? You could try to get some pictures from the rear and the front, but get down to her level. Also you could buy a wee foal testing kit, or get your vet to do a blood test, or exam.