A review of twinning in horses and the possible therapeutic value of supplemental progesterone to prevent abortion of equine twin fetuses the latter half of the gestation period.

Equine twinning, related abortion and progestogen plasma concentrations during the gestation period were reviewed. The supplemental administration of exogenous progesterone apparently prevented impending twin abortions in three mares after midgestation. A single viable twin and a mummified fetus were delivered at term by the mares.