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A couple of readers of yesterday’s post about a phantom or false pregnancy in a mare raised questions about twins in mares. So here are some observations and notes on twinning.

Horses aren’t well designed to produce successful twins, and as a result, many pregnancies that produce twins have one notably small and weak foal or two notably small and weak foals.

Either way, the breeder has a foal that is likely to be unacceptable for the sales and sometimes for racing. The latter is not always out of the question, as there are quite a few twins that have grown up acceptably and become useful racehorses.

Stop on Red, the second dam of Spectacular Bid, is one of the best-known twins. She and her sister Go on Green were by To Market out of Danger Ahead, by Head Play. Both fillies raced, each won seven races, and both became broodmares.

Go on Green was “married” to Kentucky Derby and Preakness winner Carry Back, producing four winners from five foals by that generally very modest stallion. The mare produced only one other foal, a nonwinner by Mo Bay.

Her full sister, Stop on Red, proved a much more successful broodmare, with two stakes winners and two stakes-placed runners from eight foals. Three of those black-type runners were by Promised Land, including Spectacular, the dam of Spectacular Bid.

So twinning isn’t absolutely the end of a foal’s chances to be a racehorse. (It is worth noting that Stop on Red had no reported incidents of twins, but her dam, Danger Ahead, had a set of twins that died, as well as the pair who were delivered successfully.)

Part of the consideration for a positive outcome is surely nutritional. The first hurdle for nutrition to the developing foal is that the mare can provide only about half the nutrients to each twin that she would supply to a single fetus.

Mares, for instance, have a microcotyledon placenta, rather than a macrocotyledon placenta like a cow. In addition to the difference in size, the cotyledon is part of the placenta in cattle but is part of the uterus in horses. As a result, one must never pull a retained equine placenta because of the risk of hemorrhage to the mare.

The research on equine pregnancy shows that “the primary cause of abortions in mares is still twins. If the mare gets pregnant, the foals will grow till there is not enough nourishment, then one or both will starve and be aborted. They can be asymmetrical in growth, one big and the other small. That depends on their positions in the uterus. Or you can have two sharing equally, which will usually result in abortion at eight months or thereabouts.”

The second hurdle for twins is getting enough nutrition after birth. Mares aren’t cows and don’t give an endless supply of milk. Some mares don’t even give as much as a single foal really might need, and some foals, therefore, require supplements. But the use of nurse mares and readily available milk supplements make the problem of managing a live twin much more feasible than the internal development that we still have no control over.

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