Twins in mares are considered highly dangerous, whereas multiple births in cows are generally not a major concern, says veterinary surgeon David Black from Paragon Veterinary Group. The primary reason for this difference lies in the structure and function of the equine and bovine placenta, as well as differences in reproductive physiology.
Why is carrying twins problematic for mares?
Horses and ponies have a diffuse placenta, meaning the foetal membranes must attach to a large surface area of the uterine lining to allow sufficient nutrient exchange to the foal.
When twins develop in mares, they often compete for limited space and nutrients, which can lead to intrauterine growth restriction, placental insufficiency and pregnancy failure. In most cases, one or both foals die in utero, leading to abortion, stillbirth and occasionally severe health complications for the mare. Additionally, if twins are carried to term, they are typically weak, underdeveloped and have a low chance of survival. If they do survive, the mare is unlikely to have sufficient milk to feed them both.
Additionally, horses have a much larger foetus relative to body size compared to cows, which increases the risk of complications like uterine rupture, dystocia, and post-foaling complications.
Due to these dangers, veterinarians often recommend early detection, usually by ultrasound scanning, and selective reduction of one embryo in horses, while twin pregnancies in cows are typically managed with less intervention. If multiple pregnancies are detected in a bovine, again often by ultrasound, the cow may be provided with additional food to help her maintain body condition although this can also induce larger calves.
Why are cows able to carry twins?
In contrast, cows have a cotyledonary placenta, which consists of multiple attachment points (cotyledons or 'buttons') that provide a more efficient nutrient exchange even in multiple pregnancies.
Cows are also naturally more capable of carrying twins, and dystocia (difficult birth) is less common compared to mares. However, twin births in cattle can still pose some issues, such as freemartinism, which is where a female calf born with a male twin is often infertile due to shared blood supply in utero; the male foetus develops first and hormones from the male calf pass round the shared blood supply affecting the development of the reproductive tract of the female calf.
At calving, twin calves can be presented into the vaginal simultaneously that can cause a dystocia, but these are usually relatively straightforward to correct. Despite this, twin pregnancies in cattle are not uncommon and rarely life-threatening.
Overall, twin pregnancies in mares are considered highly undesirable and dangerous, whereas they are more manageable and less risky in cows.
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