Pregnancy is an outlier in the medical world in that it is not a disease and does not, therefore, require treatment. That is not to say that there aren’t many conditions or diseases that can compromise pregnancy; there are, but failure to become or stay pregnant is not usually accompanied by obvious signs of disease.
This is inconvenient to those offering medical services and, as a result, human fertility specialists have decided that female sub-fertility should be considered a “disorder.” Of course, a disorder requires a definition, and it is now widely accepted that failure to get pregnant within a year of trying indicates that a woman, her partner, or both are sub-fertile, and that an intervention is required. While this is useful for fertility clinics, it ignores the fact that failure to establish pregnancy is often down to chance, and many cases of sub-fertility ‘resolve’ within a further year of trying, without any treatment.
If a Thoroughbred broodmare is considered an investment, failure to produce a foal in at least six years out of seven will, on average, result in financial loss.
In horse reproduction, failure to become pregnant within three cycles of mating or insemination with (semen from) a fertile stallion is considered the point at which further investigation is recommended. For an embryo recipient (surrogate) mare, failure to get pregnant after transfer of two ‘normal’ embryos is similarly considered a reason to classify the mare as problematic. Of course, at the sharp end of equine breeding, simply waiting or repeating the same management strategy and hoping is not an option, not least because of the constraints of a five-month breeding season.
There is also an obvious financial imperative; it has been estimated that, if a Thoroughbred broodmare is considered an investment, failure to produce a foal in at least six years out of seven will, on average, result in financial loss. Given an 11-month gestation, this means that a foaling mare needs to get pregnant within a month of giving birth if she is not ultimately to become an economic liability. In this light, it is not surprising that every effort is made to ensure that mares get pregnant as efficiently as possible.
Nevertheless, proving that it is necessary to instigate treatment for sub-fertility is difficult, not least because while pregnancy is a binary trait (i.e. a mare can be pregnant or not, but she can’t be ‘a bit pregnant’), fertility is much less black and white. This is more obvious for stallions, which often cover multiple mares per season. Indeed, within the population of ‘normal’ breeding stallions, pregnancy rates vary from around 35-80% of mated estrous cycles. There is no reason to assume that mare fertility does not show similar variability, but it is more difficult to quantify because few mares will produce more than a dozen foals in a lifetime. In addition, to prevent overuse of popular stallions, or escalating costs of repeated semen transport, veterinarians are expected to try to minimize the number of matings per pregnancy.
Some of the steps involved in optimizing the likelihood of pregnancy make good sense, e.g. screening mares (and stallions) to make sure that they are free of obvious reproductive pathology or venereal pathogens before breeding and careful monitoring of the estrous cycle to ensure that it progresses normally and the mare gets bred close to the time of ovulation. Thereafter, whether one should ‘do everything possible’ to help becomes ethically more challenging; some mares will benefit from the intrauterine inoculation of antibiotics after breeding or supplementation with synthetic progestogens during early pregnancy; most will not.
Sometimes it will be obvious that a mare requires additional therapy to resolve a problem, other times it won’t. Concerns about possible failure to maintain pregnancy have given rise to numerous ‘just in case’ treatments, with the assumption that therapy may help and is unlikely to harm. In this respect, while all pharmaceuticals have potential detrimental side-effects, most are either trivial or rare. However, an individual animal or body-system view of ‘it shouldn’t harm’ is under increasing pressure due to heightened awareness of the longer-term downsides of widespread antibiotic or hormone use, e.g. stimulating antimicrobial resistance, disturbing the normal microbiome (bacterial flora) and predisposing to other diseases, or compromising fertility in other species by introducing reproductive hormones into the environment.
This doesn’t mean that treatments aimed at improving the likelihood of pregnancy are always wrong, but it does highlight why research is needed into alternative ways to combat pathogenic bacteria, enhance fertility and/or more accurately identify mares that really require treatment. Progress in these directions should also help remove the specter of government regulations to limit the availability of therapeutics in veterinary medicine and/or the risk of inadvertently uncovering new diseases, or making it more difficult to treat existing ones.
These issues should prompt us to ask the question of whether failure to get pregnant should always necessarily be seen as a disease requiring a pharmacological solution; based on current research, it often is not.
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Tom A.E. Stout, VetMB, PhD
Albert G. Clay Endowed Chair in Equine Reproduction
Gluck Equine Research Center
University of Kentucky
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