Friend's horse was just like that when they bought her at an auction and then one week later her husband came running in the house saying she had a foal. My friend was in shock because she thought she was just FAT! Well anyway don't let it be a surprise be prepared always.
Thats why I never buy mares from auctions. Good luck with the baby and post lots of pictures. :) Hope its a doll. I hope all goes well and she wasn't bred to large of a sized horse that could be very damaging and riskful to her health.