Page 469 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

444 CHAPTER 2

VetBooks.ir TWINNING or hormonal induction of abortion of both fetuses.
Due to the risks of twin pregnancy to mare and foals,
Definition/overview
Multiple ovulations may be synchronous or more than any identified cases should not be allowed to continue
beyond mid-term.
24 hours apart and the resultant embryos may implant
in the same or both uterine horns. Some breeds, such Aetiology/pathophysiology
as the Thoroughbred, have a high incidence of mul- Multiple ovulations may result in the fertilisation of
tiple ovulations (20–30%) whereas twinning is rarely multiple ova at mating, which may fix in either, or both,
encountered in the pony. Some individuals and mare uterine horns. Twin pregnancies are generally unsus-
lines seem more inclined to multiple ovulations for tainable, particularly if unilateral, because of placental
their entire breeding lifetime and multiparous mares insufficiency, and they usually result in the death of
generally have a higher incidence. Twin pregnancies one or both fetuses and early abortion. They are very
are not usually sustainable and following fixation (at rarely carried to term. If they are, it is likely there will
16 days post ovulation) unilateral twins (both preg- be some fetal compromise/dysmaturity noted given the
nancies within one uterine horn) will naturally reduce shared uterine and placental space. Mummification of
to a singleton in 85% of cases. In bilaterally fixated one fetus and the pregnancy continuing to full term
cases (one pregnancy in each horn), the reduction rate and producing a normal foal has been recorded.
is 15% (Fig. 2.51). Prior to routine ultrasound moni-
toring this incomplete reduction of twins led to sig- Clinical presentation
nificant mid-late pregnancy loss. Identical twinning, Two or more ovulated follicles are palpable on a rou-
due to splitting of a single fertilised ovum, is extremely tine post-ovulation rectal examination or ultrasound
rare in the horse. Diagnosis of twinning is by ultra- scan, or two conceptuses at a later stage. Early abor-
sound examination and manual palpation. Evaluation tion may occur, or birth of two dysmature or dead
of every mare at pregnancy examination for twin foals at term, if the pregnancy is allowed to con-
pregnancy is a core part of modern stud farm man- tinue. Abortion is usually later (7 months) for bilat-
agement. Treatment is by manual rupture of one fetus eral twins. Mares may also present with abnormal
abdominal enlargement, pre-pubic tendon rupture/
ventral wall rupture and ventral oedema.

2.51 Diagnosis
A thorough examination of the entire reproduc-
tive tract using ultrasound scanning at 14–15 days
post ovulation is the best time to identify the still
mobile embryos and multiple CL (Fig. 2.52).
Assessment of the ovaries for follicles and ovula-
tions during the breeding cycle will highlight mares
that must be checked for twins at 14–15 days. High-
quality equipment should be used and if twinning
is suspected, but not identified, the examination
repeated in 24–48 hours to confirm the findings.
Asynchronous ovulations may result in different
sized embryos that are missed if only one examina-
tion is carried out. Endometrial cysts can appear
Fig. 2.51 A remarkably uncommon situation in similar to the early conceptus, and detailed record
which twin foals have been born live to a pony mare keeping (of previous examination results) or
and survived. Both were dysmature at birth. Note the re-examinations are necessary to confirm that the
disparity in the size of the two foals. cyst is not an embryo. Unlike endometrial cysts,