Miscarriages in Horses Provide Clues to the Causes of Early Pregnancy Loss in Women

A study of horses – which share many important similarities with humans in terms of their chromosomes and pregnancies – found that 42% of miscarriages and spontaneous abortions in the first two months of pregnancy were due to complications from an extra set of chromosomes, a condition called triploidy. During this embryonic period [up to eight weeks after conception], triploidy has rarely been observed in mammals other than females. The study shows that this is probably the main cause of pregnancy loss after natural conception in the first six weeks of gestation.

Horses Are a Good Model for Studying Human Pregnancies

Miscarriages in humans occur in 10-20% of pregnancies and are often associated with chromosomal defects, but until now there have been no suitable animal models that truly mimic the characteristics of this condition. The new research findings will help veterinarians better understand the causes of pregnancy loss in horses and make horses an excellent model for studying human miscarriages.

For the study, de Mestre’s lab at Cornell, and previously the Royal Veterinary College in London, obtained 256 fetal and placental samples from veterinarians who had treated horses with failed pregnancies over a 10-year period. Using the samples, the researchers were able to investigate the prevalence of different types of chromosome copy number errors associated with pregnancy losses. They found that chromosomal errors were associated with 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 between days 111 and 110. Aneuploidy (loss or gain of a single whole chromosome) was mainly associated with miscarriages in the first 10 weeks of pregnancy, while deletions or duplications of only part of a chromosome were found in miscarriages after 110 days. These results are strikingly similar to those observed in a number of large studies in women, the study says.

Miscarriages: Investigation of Chromosomal Errors

Horses are a good model for studying human pregnancies because they have a similar gestation period – 11 months compared to nine months in women – and the embryo develops at a similar rate in the early stages. In addition, equine chromosomes have a very similar genetic content to human chromosomes, making them particularly relevant for the study of chromosomal defects.

The reasons for miscarriages in women in early pregnancy have previously been difficult to determine, as most fetuses are lost at home during this time, leaving scientists with no material – and no data – to study. The results of the study shed light on the frequency of chromosomal defects in the period corresponding to the first six weeks of human pregnancy. Because of the value of horses and the emotional attachment their owners have to them, horses are very well cared for, with pregnancies routinely tracked, which in turn provides extensive data for research.

Other animal models, such as mice, are not comparable to human pregnancies. Mice have a gestation period of about three weeks, and natural pregnancy losses in mice are low. In terms of equine health, the study provides new details about common chromosomal abnormalities that are likely to change the clinical management of pregnancies. For example, if a clinician determines that a horse has a severe chromosomal abnormality, he or she may no longer choose to prolong pregnancy by administering hormones, as is common in pregnant mares. The study will also pave the way for researchers to develop new diagnostic tests for chromosomal abnormalities in horse fetuses and investigate the molecular mechanisms that lead to these abnormalities.