Morphology of twin and triplet equine conceptuses during weeks 3 and 4 of pregnancy.

The research examines how twin and triplet horse embryos develop during the third and fourth week of gestation, providing an insight into how and why ’embryo reduction’ occurs naturally in horses, meaning how twin pregnancies are largely reduced to singleton pregnancies.

Introduction and Methodology

• The research begins by highlighting the curiosity that twin ovulations are common in horses, but twin pregnancies in this species seldom reach full term. The study aims to provide a morphological explanation to this common phenomenon known as ’embryo reduction’, where one or both twins are naturally eliminated in early pregnancy.
• Previous theories have been based on ultrasonographic studies and this study now attempts to observe and describe the physical and structural development of the horse conceptuses.
• The researchers chose 31 twin pregnancies and two triplet pregnancies, with observations conducted from Day 15 to Day 28 of the pregnancies. The observations were made through the retrieval of the conceptuses transcervically (through the cervix).

Observations and Findings

• The study found signs of contact between conceptuses, which they inferred from one pair that remained attached by their capsules on Day 18. Interestingly, ultrasonography had not revealed these signs of contact.
• At Days 16-17, signs of adhesion between pairs were found, which strengthened by Day 18 in nine of the unilateral pregnancies.
• The observed adhesions between the twins significantly disturbed the vitelline circulation and in some cases, caused degeneration in one of the twins.
• By Day 21, five out of seven unilateral twin pregnancies documented signs of embryo reduction. In the two surviving pairs, the attachment between the twins was located near the trilaminar/bilaminar yolk-sac wall border.

Conclusions

• The research concluded that the biological capsule of the embryos initiates adhesion between twins — in the early days of the pregnancy.
• It also identifies the crucial role of a functional vitelline circulation and an unencumbered trilaminar yolk-sac wall for twin survival. This is a critical morphological insight offering an explanation to why horses rarely carry twin pregnancies to term.