Incorrect. This is true for dogs, cattle and maybe other livestock, but horses are different. In the equine the foal only grows to the extent it has room in the uterus. After birth, the genetics kick in, and the foal will grow according to that.
Not always true, and via the Shire/Shetland study, the Shire foals ET'd into the Shetland mares did not grow as big as the Shire in Shire foals did.
As well, the other study posted here clearly showed a relationship between the larger foal in the smaller mare being born with many of the pre/dysmature symptoms and clearly being less developed than the smaller-in-larger and same-size foals. Thankfully, for our knowledge, those dysmature foals corrected after some time, but some did require extra care in the beginning. But, at least with those, those were big horse foals in a pony mare, not a genetic mix, so it's not fair to extrapolate that information directly to this particular situation. Still, it's good info to be aware of.
______________________________ The CoTH CYA - please consult w/your veterinarian under any and all circumstances. - ET
Have just been informed that a clients mare foaled a healthy filly foal without problems this am. The colt is 16.3hh and the mare a Welsh Section A. The mare and colt shared the same pasture, not sure what they were thinking but thankfully all is well.
And here is more interesting food for thought on the uterus issues. Not a controlled study, but we know of a 16 hand stallion that was bred to mares 14.2-17 hands. Only about one hundred foals, but out of them, there was a huge incidence of shoulder-locked foals and difficult births. But they were all out of the 16.2 plus mares! Not a single one out of the smaller mares.
I don't think any of the mares or foals were lost, but nerve-wracking experiences and close calls getting these foals out.
So it made us wonder if these big mares had big uteruses that grew foals that were just too big to get through the birth canal? Obviously, one had to look at the stallion too since he is the one who consistently added a couple of inches to the fully grown offspring. The 14.2 hand mares had 15 hand offspring, the 17 hand mares had 17.2 hand offspring, etc..
But what was interesting is that the little mares seemed to have the ability to "shrink" the foals to fit, while the over 16 hand mares really had a hard time with this particular stallion.
And most of the big mares owners had spent their previous year fretting about whether or not their foal would be big enough since the stallion was "only" 16 hands, go figure!
So the lesson that we learned here is that you really never know how your mare is going to produce until you breed her. And many old timers swear that the first foals are the smallest (and stay the smallest), no matter the stallion, which makes one think that the uterus size is making a difference there too. Not to mention twins which never catch up to their genetic potential due to being crowded.
Interesting discussion, and once again, all the best to the OP and her exciting news!