How, um, LARGE are your preggo mares?
One of mine is ginormous, the other just rounding out a bit. They were bred within two days of each other and are 6 months along. The huge one is 17 yrs old, her fifth foal, last one born in 2007, big frame, longish back. The other is 12 yrs old, second foal, last one born in 2001, big bodied, but refined. Both are about 16 h., older one is full Trak, younger one is half Trak/half TB. Is it just the difference in number of previous pregnancies? An indication of how big the foal might be? The big one is definitely (according to vets) having a colt, but I didn't have the other one checked for sex because SO didn't require it and I was trying to save $$$'s. Now wishing I had, but name picked out is unisex, so whatever it is, it is and I'll be thrilled as long as it's healthy and mare does OK.
Her first foal was born dysmature, but developed normally and has no problems as an adult (other than your usual chestnut mare issues). :)
Found a pic from a month ago