It is well known that the mare’s uterus is not well equipped to handle twin pregnancies to term. Most twin pregnancies result most commonly from double ovulations that are either synchronous (ovulate at the same time) or asynchronous (ovulate at different times) with as much as 96 hours separating the two ovulations. When asynchronous ovulations occur, especially those that are many hours apart, the two resulting embryos can be of very different sizes, making it a challenge to detect them during those early ultrasound examinations.
The goal of this study was to (1) document the incidence of synchronous and asynchronous ovulations and (2) to determine whether this varies with month/season. 506 cycles from double ovulating mares were monitored at 8+1hour intervals until 96 hours after the first ovulation. Mares were grouped according to the 8-hour ovulation interval and month/season. Analysis of the results showed that asynchronous double ovulation is common and more prevalent at the very beginning and end (transitional phases) of the normal breeding season. It “emphasizes the importance of closely monitoring mares, especially at the extremes of the breeding season, for double ovulations up to or at 96 hours post initial ovulation in order the minimize the chance of missing multiple pregnancies.”
Relevance? It has long been recommended that the first ultrasound for pregnancy be done around day 13-14 post ovulation and again at around day 20-21 after ovulation to check the initial embryo and check again for a second younger embryo. It is also important at this first ultrasound to not only examine the uterus for embryonic vesicles, but to also closely scan the ovaries to determine if more than one ovulation occurred. It a second embryo is detected, it can still be successfully reduced if the two vesicles are in different uterine horns or can still be separated from each other and moved far enough apart to reduce one of them.
Scientific Article: Morel, MC, Newcombe JR, Reynolds, N. “Asynchronous ovulation in mares: seasonal variations in frequency”. Vet Rec. 2015, March 21: 176 (12), 310. Epub 2015 Jan 23
Companion Article: Oke, Stacey DVM “Double Trouble: Multiple Ovulations in Mares”, theHorse.com Apr 04, 2015. Article #35559.