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Bilateral Twins


Compiled by Leigh Ballard

Since twins are a rarity in the horse world, we decided to explore further how twin conceptions happen. Here is information gleaned from Yallambee Stud, located in one of Australia’s most prolific regions for breeding sound Thoroughbreds.

“Twin conceptions in the horse are very rarely derived from the division of a single fertilized ovum. Multiple ovulations, either in the same or in separate ovaries, are most often responsible for the incidence of twin conceptions

“In a normal pregnancy, the embryo is mobile from day 9 through to day 16. At day 16 it will 'fix', or 'settle' either side of the junction of a uterine horn and uterine body (corpus corneal junction). This is referred to as Embryo Fixation. The same applies if there are two conceptions. Both embryos may fix in the same horn (Unilateral fixation) or, at separate horns (Bilateral fixation). It is not until this stage that a mare is likely to naturally terminate one of her pregnancies, with a high proportion of an embryo discontinuing development between 25 and 40 days.

“Unilateral fixation is more common than bilateral fixation. Unilaterally fixed embryos are more likely to naturally reduce to a single viable embryo by day 36 - 40 than bilaterally fixed embryos. This is thought to be the result of the two sets of membranes coming into contact and competition for nutrients.

“Bilateral fixation will result in fewer twin conceptions reducing to a single embryo by day 40. These are capable to surviving longer because it is not until they become more developed that the real competition for uterine/placental space - hence nutrient availability - comes into play.

“As pregnancy progresses beyond day 40, there is increasing pressure for the embryos to compete for the available uterine wall space. One of two scenarios may develop during a twin pregnancy:

1.      Symmetrical distribution of the placentas – where the two placentas share equal portions of the uterine wall. This may result in two live foals being born alive and at full term, but is quite rare.
Very rarely does it eventuate that two healthy twin foals are born alive.

2.      Asymmetrical distribution of the placenta – where one fetus successfully out contests the other for more of the uterine wall.

“If both are born alive, one of the foals is likely to be significantly undersized. Vital organs and bones are unlikely to be fully developed in most live twin foals, hence survival rates are limited. Even a quite well developed, live, twin foal is likely to be very open to infection, and possibly have slower development.

“The actual birth of twins can often be complicated by presentation problems, causing future reproductive problems to the mare.”


Yallambee Stud, “About Twins in Thoroughbred Breeding.”
Goulburn Valley Equine Hospital:
The, “Seeing Double,” May 16, 2014
KE Wolfsdorf and ML Macpherson, “Management of Twins,” Hagyard Equine Medical Institute, Lexington, Kentucky.
West VETS Equine Reproduction Center, “Double Trouble: Twin Pregnancies,”
Dr. Mina C.G. Davies Morel, University of Wales, Aberystwyth, “Twin Pregnancies, A Problem in the Mare.
Patrick McCue, DVM, PhD, Colorado State University Equine Production Laboratory, “Twins.”

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