Improving the odds: Pinpointing the key factors in Thoroughbred pregnancy loss

Last month, we looked at promising research into the use of niacin to improve pregnancy rates in older mares, and this month we turn our attention to keeping mares pregnant - and what happens when a mare loses her pregnancy. By understanding the reasons that may cause a mare to abort, we can move towards improving the nation’s foal crop at the source.

AgriFutures Australia’s Thoroughbred Program has been undertaking a multi-stage project, The pathology and epidemiology of equine pregnancy loss, to enhance the industry’s knowledge of what causes abortion, and as they enter the next phase, they expand the project’s reach to encompass the breeding industry in Queensland.

Understanding the risk

Despite a steadily declining number of Thoroughbred mares covered in the last two decades, the percentage of live foals born from those coverings has risen with improvements in reproductive science and management techniques. According to data released by the Australian Stud Book, that percentage has sat around 65% for the past five seasons.

Breeding is a costly and time consuming endeavour, and one key way to improve outcomes after a mare has been covered is reducing risk factors in the mare’s environment. The risks associated with birds, bats, and processionary caterpillars are well known, but at what stage in a mare’s pregnancy is she most at risk and what factors lead to her being most exposed?

The answer varies by region, so in 2020, AgriFutures Australia’s Thoroughbred Program embarked on a body of research to better understand the circumstances around pregnancy losses in different areas of the country. The first stage examined data from a breeding season and the subsequent pregnancies in the Hunter Valley with the intention of understanding what were the leading causes of pregnancy loss and how were those losses distributed.

“There was not a lot known about the epidemiology of pregnancy loss,” said Dr Joan Carrick of Equine Specialist Consulting. Leading the project alongside the University of Sydney’s Associate Professor Victoria Brookes, Dr Carrick specialises in perinatal foal pathology and managing high risk pregnancy, and has brought over four decades of knowledge to the project.

“It can vary a lot from farm to farm and year to year. We performed this study to try and further our understanding of when and where losses occur, and what were the most important aspects of those losses for the farms.

“While nobody likes to talk openly about losses, when farms have problems or outbreaks, they do often confidentially reach out to one another and keep each other informed. We wanted to get a broader picture of what was happening across the whole region. It was really important to gather all of the additional information, and then it would be lovely to follow that up year on year.”

The impact of environmental factors

The initial study, published in April 2025, retrospectively looked at the 2021/22 season in the Hunter Valley, examining not just information about the mares themselves, but also about the environment that the mares were kept in, and stud practices for biosecurity, pest control, and feeding. Mares submitted for the study were divided into two groups; case mares who experienced pregnancy loss, and control mares who did not.

Two things were quickly identified; control mares were six times more likely to be vaccinated against salmonella, and there was a relationship between environmental factors of paddocks themselves and rates of mares aborting.

“We collected information about the paddocks that the mares have been in,” Dr Carrick said. “What sort of trees, what sort of pasture, what the water source is like, whether it has a single fence or double fence, if it’s near roads, near bushland, all those sorts of things. We certainly found that there were some significant differences in the paddocks that the mares were more likely to abort.

“The mares that were more likely to abort tended to be kept in the central areas of the farm where the paddocks are less manicured. That makes sense, actually, because the studs are probably selecting those mares and watching them more closely - they're near the crushes, they're not kept at the front of the farm, where all the flash fencing is - but we don't know for sure just yet. We need more data.

“So again, we're looking at the environment around these mares and trying to get some indication as to what contribution it makes.”

Paddocks with non-native tree lines or with trees well fenced off from mares were less likely to contain mares who aborted during the study’s limits, as were those close to staffed buildings or the road. It indicated a need for further research into the relationship between these elements and factors causing pregnancy loss.

Another study under the same project umbrella examined data from fetal loss submissions in the Hunter Valley between February 2015 and November 2021, and observed several patterns in the causes for pregnancy losses over the period. Adding environmental data to the findings brings the picture of abortion more into focus.

“I think if we find the same sort of pattern again with the environment, then I think our next question is, ‘well, what is it exactly about those paddocks and how the mares are managed in those paddocks that is contributing to them aborting?’,” Dr Carrick said.

“You often end up asking more questions than you answer with any research, but I do think the environmental information is unique. No one has ever tried to collect it before as it’s quite difficult, but it is crucial to understanding the whole picture.”

Mapping the trends

The current study, which kicked off at the beginning of 2025, is examining the same factors again across the Hunter, and has also expanded to include studs in Queensland, with the same case-control format to how mares are recorded for the study. Dr Carrick emphasised the importance of repeating the research across Australia’s different breeding regions. Little work has been done to understand losses in Queensland in particular.

“It’s really good to expand to Queensland as the environment and climate is different again from the Hunter Valley,” she said. “The trends we will see are unique to each region. For example, there was a large study done in the United Kingdom with thousands of mares that showed that mares that have a late cover date or take multiple serves to get pregnant are more likely to lose their pregnancies. Within our project, we could see a tendency for that to happen, but it didn't come out as significant in our analysis, which is why it is important to repeat the study on a larger scale to see whether it is real in Australia.

“We can’t just extrapolate from studies done in the UK or the States as the environment is so different, and it is the same between regions in Australia.”

Repeating the study in the Hunter would allow the research team to build on what they had learned in the previous study, and see whether the patterns they observed then were any different.

“Collecting data this year will be very helpful as we can compare it year on year and compare the patterns and trends that we are seeing,” said Dr Carrick. “I have been observing this for the last 25 years and there are definitely patterns to outbreaks in different years, so understanding those different patterns is quite important.”

A roadmap for the future

The intention for the current study is to build on the data already collected and analysed, and move towards formulating strategies to minimise the risks identified.

“The studs that have participated have been very supportive of the project, so hopefully we'll have all that data collected by the middle of February and be able to start analysing it,” said Dr Carrick.

Information about pregnancy losses in Queensland will be made available to breeders in the state so they can better understand the factors influencing pregnancy loss in their environment, the number of pregnancy losses experienced in the region, and when those losses happen. Knowing when to be most vigilant will help breeders allocate time and resources most effectively.

“Every pregnancy is precious,” said Dr Carrick. “You have worked hard to get them to that point, so you want to keep them there. The other aspect to it is that, if you do have a compromised pregnancy - if the mare is compromised, or there's poor bloodflow to the placenta - the foal that is developed often comes out a little bit backward.

“But what is really key and has been shown in most other species is that those animals often go on to have some long term health problems, but also they don't get to the same level of athlete as their genetic potential. Organ development, particularly the heart and lungs, is compromised during those kinds of pregnancies, and that's so crucial for breeders. We are trying to breed racehorses after all.”

AgriFutures Australia has released a resource package on preparing and managing mares as we enter abortion season.

Dr Carrick hopes that the research could shift the needle on the ratio of covers to live foals born every year, by assisting more pregnancies to go to term.

“We want every pregnancy, not only to go full term, but to be a healthy pregnancy that produces a healthy foal every time,” she said. “That is aspirational, but what we need to do is understand what causes the losses so we can minimise any factor that contributes to the cause of losses. There will always be genetic factors and influences that we can't do anything about, but if we understand the factors that we can do things about, then we can reduce our losses.”