A recent study focusing on horses, which share key similarities with humans in terms of chromosomes and pregnancies, revealed that 42% of miscarriages and spontaneous abortions within the first two months of pregnancy are due to triploidy—a condition where an extra set of chromosomes is present.
“Up to eight weeks from conception, triploidy had rarely been reported in mammals outside of women,” said Mandi de Mestre, a professor of equine medicine at Cornell University. “Our study shows that in the first six weeks of gestation, triploidy is likely the main cause of pregnancy loss following natural conception.”
De Mestre is the lead author of the study titled “Naturally Occurring Horse Model of Miscarriage Reveals Temporal Relationship Between Chromosomal Aberration Type and Point of Lethality,” published on August 5, 2024, in the Proceedings of the National Academy of Sciences.
While human miscarriages occur in 10-20% of pregnancies and are often linked to chromosomal errors, there has been no suitable animal model that accurately replicates this condition. This new research highlights horses as an excellent model for studying human miscarriage, offering insights that can help veterinarians better understand and address pregnancy loss in horses.
“We were able to study the impact of chromosomal errors throughout the entire pregnancy in horses,” said de Mestre. “We discovered that triploidy is linked to early pregnancy losses.”
De Mestre’s team at Cornell University, along with earlier work at The Royal Veterinary College in London, analyzed 256 fetus and placenta samples collected over a decade from horses with failed pregnancies. The researchers examined the prevalence of different chromosomal copy number errors associated with pregnancy loss. They found that chromosomal errors were present in 57.9% of pregnancy losses up to day 55 of gestation, 57.2% of losses between days 56 and 110, and only 1.4% of losses from day 111 to the end of pregnancy.
Aneuploidy, the gain or loss of a single whole chromosome, was primarily associated with miscarriages within the first 10 weeks, while partial deletions or duplications of chromosomes were linked to miscarriages after 110 days. These patterns closely mirror those observed in large human studies.
Horses make a good model for studying human pregnancies due to their similar gestation period—11 months compared to nine months in humans—and the comparable rate of early embryonic development. Additionally, horse chromosomes have a similar genetic content to human chromosomes, making them particularly relevant for studying chromosomal errors.
Determining the causes of very early miscarriages in women has been challenging because most fetuses are lost at home during this period, leaving scientists without material to study. This study offers valuable insights into the frequency of chromosomal errors during the equivalent of the first six weeks of human gestation.
The high level of care and routine pregnancy tracking in horses, driven by their value and the emotional attachment of their owners, provides extensive data for research, unlike other animal models such as mice, which have much shorter gestation periods and low natural pregnancy loss rates.
In terms of equine health, this study provides new information about common chromosomal abnormalities that will likely influence clinical management of pregnancies. For instance, if a major chromosomal error is detected, clinicians may decide against extending the pregnancy with hormone treatments, a common practice for pregnant mares.
The findings also pave the way for developing new diagnostic tests for chromosomal abnormalities in horse fetuses and exploring the molecular mechanisms leading to these abnormalities.
“This research has laid the groundwork for understanding the genetic causes of pregnancy loss in horses, often referred to as pregnancy loss of unknown cause,” said Shebl Salem, a postdoctoral researcher in de Mestre’s Equine Pregnancy Lab and a co-first author of the study.
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