Page 942 - The Veterinary Care of the Horse

• Twin pregnancies (see below).

VetBooks.ir Early embryonic loss may be predicted from the appearance on the ultrasound scan. The

signs include:
• an embryonic vesicle that is smaller than expected for its age

• a vesicle with no embryo visible after the time when it should be seen

• an embryo that is smaller than expected for its dates.

TWINNING

Nearly all twin pregnancies result from double ovulations. These may occur close together or
several days apart within a single oestrous period. The incidence of double ovulations differs

between breeds and increases with the age of the mare. Thoroughbred mares are reported to
have a 15–25% occurrence of double ovulations with up to 15% incidence of twins.

Twinning is a serious source of loss to the breeding industry. Various reports suggest that
between 53% and 73% of affected mares will abort and of all the mares with twin

pregnancies, only 16–25% of mares will give birth to single or twin foals. The cause of
embryonic loss is the inability of the endometrium to provide adequate nutrition for both
embryos (Figure 19.19). Twins located in the same horn are likely to die earlier in the

pregnancy because the vesicles are in contact with each other rather than with the lining of
the uterus and their nutrition is reduced.