Increased Incidence of Twins

If research shows that Thoroughbreds and Warmbloods are more likely to have twin pregnancies, could this be in relation to the number of mares that receive injections to encourage ovulation?
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Increased Incidence of Twins
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Q. I read the article about twins (“Seeing Double”) and would like to ask a question: If research shows that Thoroughbreds and Warmbloods are more likely to have twin pregnancies, could this be in relation to the number of mares that receive injections (of ovulation-inducing agents such as human chorionic gonadotropin or deslorelin acetate) to encourage ovulation?

I allow my mares to breed naturally (live cover rather than by artificial insemination) with no interference apart from ensuring a clean uterus prior to breeding. To date I’ve not had any twin pregnancies. I would love to know if the injections can cause the release of two eggs around the same time.

Evelyn Williamson, New Zealand


A. This is an interesting question. In fact, there have been at least a couple of studies that looked at whether inducing ovulation could contribute to increased incidence for twins. It is logical that the risk for twinning will increase if there are two follicles present and we encourage both of them to ovulate by administering hCG or deslorelin. Yes, there is a risk that subordinate follicles that would normally regress over time would be induced to ovulate. This is why when managing mares with hormone treatments or otherwise, it is very important to determine the ovaries’ follicular situation and follow up with examination after ovulation

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