So I have always seen on other forums posts about how a mares system doesn't allow a foal to grow to large to be born. So why does it happen?
I purchased a mare last early winter. She was 9 months along. She is a sturdy 16.2 mare, 1300# she was bred to an equally sturdy 17hh Shire cross. The mare herself is part draft.
Normal pregnancy, she went into labor at day 352. The front feet presented but no progress was made, called the vet since the mare decided 11am was a good time to foal, the vet was hours away at a farm visit. Scrubbed up reached in and found the muzzle was trapped between the front legs and tried to reposition foal. There was absolutely no room. Tried for a long time on my own since calls to 4 other vets resulted in the same out of town answer. Ultimately I did get a vet and a trailer out at the same time. By this time (hours later) the foal had passed and the efforts were to save the mare. Decided to load her up and take her to Cornell Veterinary. A fetonomy was performed to remove the 150# (YIKES) filly. They weren't even sure after removing the head if the shoulders would pass. after foaling, she was in ICU waiting for the placenta to pass....it took 5 days. Thank goodness she never went septic or foundered. (She wore ice boots and an iv of antibiotics constantly until 24 hours after passing)
The hauler (a local wb breeder) commented that she had never seen hooves that size on a newborn...heck she had yearling connemara's with smaller feet.
So what could have caused the foal to grow so large? Mare wasn't on feed until she came here and then it was minimal until the last few weeks, and even then it was 16% and less than 4qts a day. (she lives on air...)
Vets said all is well with the mare, no tearing or damage and she is breeding sound. I just wondered what could be done in the future to prevent this from happening again, should I ever get the guts to breed her again. I could rationalize it in my mind if the sire and her were different body types and he was hands taller, but then everyone says drafts can be bred to ponies.... ideas?
The uterus controls the size (height and mass) to which the foal grows, but may not have reliable control over width and depth of chest and hips, causing problems when a narrow-pelvic-opening is encountered. This is typically where "size" issues are found.
What you describe actually is more of a dystocia situation, which was probably coupled with an exhausted mare unable to push adequately after the 4 hours that you worked on the mare before she got to Cornell.
There is one exception to the uterine-control rule, although I doubt that it applied in this case, and that is if endophyte exposure is encountered. This can lead to abnormally prolonged gestational duration, with the fetus continuing to grow in utero. As I say, I doubt that applies here, as 352 days is well within the "normal" range of 320-370.
One also has to consider that animals are not computers, so there are always exceptions to "rules"!
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