Examining the Mare's Placenta
and Keeping Foaling Records
Table of Contents
- Placenta of the Mare
- Foaling Records
The placenta from each foaling mare should be examined for completeness
shortly after foaling (1). If a section(s) of placenta remains in
the uterus, infection (endometritis, pyometra) and toxemia (resulting
in laminitis) may result. Persons inexperienced at looking at the
mare's placenta may mistake normal structures as lesions.
The mare's placenta is made up of two distinct membranes, the amnion,
the opaque membrane which immediately surrounds the foal, and the
chorioallantois which joins to the uterus. On initial examination,
one first encounters a placenta which has been turned "inside-out".
The "velvety" chorionic surface will be inside while the
glistening allantois surface is outermost. This inversion occurs
when the foal, still enveloped in the amnion, breaks through the
chorioallantois at the cervical star. The foal moves into the cervix
and vagina pulling the chorioallantois by way of the umbilical cord,
everting it through the hole at the cervical star. The cervical
star is a pale area of the placenta which apposes the cervix. It
lacks the finger-like villi which give the chorioallantois a velvety
Examination of the placenta starts with obtaining an accurate weight.
Digital postage scales, capable of weighing up to 30 lbs., are inexpensive
and work well. The normal placenta of the near-term thoroughbred
mare should weigh about 11% of the body weight of the foal or approximately
5.7 + 0.08 kg (12.5 lbs) (2). Placenta weights are important indicators
of edema and reactions to infectious and non-infectious agents that
may affect foal mortality.
Placenta of the Mare
(chorioallantois - chorionic surface)
While the glistening allantois surface is still outermost, check
for any tears or any abnormalities. The most common area for tears
is the tip of the non pregnant horn. Once assured that the placenta
is complete, turn it "right-side" out, exposing the "velvety"
chorionic surface. Lay the placenta out in a "Y" or "F"
shape. The 3 arms of the "Y" correspond to the placenta
occupying the 2 uterine horns and the equally long uterine body
ending in the cervical star. This is usually the point of rupture
through which the foal exits.
Examination of the normal placenta may reveal slight edema and/or
thickening of the tips of the uterine horns, small (0.1 - 1.0 cm)
bare foci scattered at random over the chorionic surface, and distinct
folds of the chorionic surface of the non pregnant horn. These folds
are prominent since they were not stretched by the pregnancy. Often
a brown "liver-like" material, termed a hippomane, is
found floating in the allantoic cavity. Dr. Jim Rooney theorizes
that the hippomane arises as an out-pouching of the allantois which
eventually forms a pedicle and separates to float free in the allantoic
cavity (3). Other references suggest that it is a collection of
debris, thought to be deposits from the foetal urine and cells (4).
Histologically, it appears to be a concentric deposition like a
The umbilical cord should be measured. The umbilical cord of the
thoroughbred horse is usually between 36 and 83 cm in length. Some
have been reported up to 105 cm in length. However, the greater
the length of the umbilical cord, the greater the chance for umbilical
torsions. It is common to see five or six twists of the umbilical
cord with no apparent problem, while 16 twists have been associated
with death of the foal. Local haemorrhage and edema of the umbilical
cord are observed in some cases.
The presence of yellowish brown meconium staining of the placenta
and foal should be noted. It indicates fetal stress, as seen with
herpesvirus abortions, ergot alkaloid toxicity and other diseases.
Your veterinarian should be called to examine all post-partum mares,
foals and placentas, especially if a placenta is not intact. Practitioners
may choose to submit the entire placenta for examination to a laboratory,
or section parts of the placenta and place them in formalin. Refer
to the foaling record for selection of areas to be sampled. Samples
may also be collected for bacterial culture. Entire placentas should
be packed with ice and shipped in sealed watertight containers to
the laboratory. Small plastic pails with tight fitting lids work
well for this purpose.
A record of events and time intervals between events is very useful
information. A foaling record provides a systematic record of events
and can be copied to the laboratory when submitting samples. Accurate
records can be used to investigate or determine the cause of foaling
problems within a group of mares during the foaling season and can
also be useful in determining what is normal for individual mares.
Industry bench marks for these events and intervals are summarized
below (5, 6).
- The average gestation length of a mare (period between breeding
and foaling) is 340 + 20 days.
- 65.5% of mares foal at night between 8 p.m. and 1 a.m.
- Waxing of the teats occurs 2-4 days before foaling. Waxing (or
wax beads) refers to the colostrum (first milk) which appears
at the end of the teats.
- Dripping of colostrum 24-48 hours before foaling (may result
in inadequate anitbody absorption in the foal).
- Stage1 lasts for 2-3 hours and terminates with the "breaking
of water" when 2-5 gallons of uterine fluids are released.
- Stage 2 is the active parturition stage, when the mare's cervix
dilates and uterine contractions increase with the foal normally
expelled in 15 minutes.
- Stage 3, the final stage, involves the expulsion of placental
membranes. The placenta is usually passed within 15 minutes to
- Placenta weight 5.7 + 0.08 kg (12.5 lbs) (2)
- The umbilical cord of the thoroughbred horse is usually between
36 and 83 cm in length. Some have been reported up to 105 cm in
- It is common to see five or six twists of the umbilical cord
with no apparent problem, while 16 twists have been associated
with death of the foal.
1) Schlafer DH. Gross examination of equine fetal membranes:
what's important - what's not. Proc. of the Equine Symposium:
The Perparturient Mare and Noeonate, Society of Theriogenology/American
College of Theriogenologists, San Antonio, Texas. Nov. 2000.
2) Whitwell KE, Jeffcott LB. Morphological studies on the fetal
membranes of the normal singleton foal at term. Res. Vet. Sci.
3) Rooney JR, Robertson JL. Equine Pathology. Ames: Iowa State
University Press, 1996: 240.
4) Dickerson JW, Southgate DA, King JM. The origin and development
of Hippomane in the Horse and Zebra. J. Anatomy 1967; 101 (2):285-293.
5) Freeman D.W., Oklahoma State University, Equine Specialist.
6) Ley W. B., Management of the Foaling Mare: Predicting Readiness
for Birth and Inducing Foaling. Veterinary Practice, June 1994