The twinning connection.

The research article attempts to outline the causes and patterns of twin pregnancy in mares, and explores how certain measures may counteract its negative implications on fertility. The researchers find that twin pregnancies mostly result from multiple ovulations occurring at different times. They also present an argument against the common avoidance of mating mares with potential twin ovulations and describe some methods that can limit the occurrence of twin pregnancies.

Twin Pregnancy in Mares as a Fertility Issue

• The paper starts by discussing the significant problem of twin pregnancy in mares, especially in Thoroughbred mares, where it occurs in about 3 per cent of pregnancies.
• Twin pregnancy can lead to horse wastage as both fetuses might abort or fail to survive birth. It also often leads to reduced fertility in subsequent years.
• Owners and breeders often abstain from mating mares which show signs of twin follicles to avoid the issues related to twin pregnancy.

Characteristics and Causes of Twin Pregnancy

• The article asserts that twinning in mares is generally a consequence of multiovular twinning, with more than one ovulation happening during or near oestrus. It, however, points out that there is a clear discrepancy between the rate of twin ovulations and the actual incidence of twin pregnancies.
• The study mentions that twin ovulations happening synchronously (on the same or successive days) rarely result in twin pregnancies, even if both ova get fertilised. Such cases even lead to a higher singleton pregnancy rate than in mares with only a single ovulation.
• Contrarily, asynchronous ovulations (twin ovulations happening with a few days gap) tend to result in a high percentage of twin pregnancies. The research dispels the earlier notions about the inhibitive effect of progesterone levels on conception during dioestrus, based on the observation that younger conceptus can coexist with an older one from the previous oestrus.

Managing and Detecting Twin Pregnancies

• Given the limited success of managing strategies to prevent twinning, other solutions like the manual rupture of one fetal sac have been attempted, which is reportedly effective if conducted before Day 31 after ovulation.
• However, early detection of twin pregnancies is difficult, especially if the two conceptuses lie adjacent in the same uterine horn.
• Ultrasonic echography has shown promise in the early detection of twin pregnancies, but some twins still go unnoticed.

Potential Factors for Twinning and Gonadotropin Levels

• The researchers list some potential factors for twinning that are observed in other species, such as genetic factors, improved nutritional status during the breeding season, and sexual stimuli that increase plasma luteinising hormone (LH) concentrations.
• The use of exogenous gonadotropin administration to increase the ovulation rate in various species is discussed. In mares, administration of LH or the LH-like human chorionic gonadotropin (HCG) does not appear to be associated with twinning.
• The study also explores the plasma gonadotropin levels in mares with single and double ovulations, but it highlights the huge variability between individuals which makes statistical differentiation between single and twin ovulations difficult.