Examining the mare’s placenta and keeping foaling records

Introduction

The placenta from each foaling mare should be examined for completeness shortly after foaling^{footnote 1[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 appearance.

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)^{footnote 2[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

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^{footnote 3[3]}. Other references suggest that it is a collection of debris, thought to be deposits from the foetal urine and cells^{footnote 4[4]}. Histologically, it appears to be a concentric deposition like a urinary calculi.

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.

Foaling records

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^{footnote 5[5]}, ^{footnote 6[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 one hour.
• Placenta weight 5.7 + 0.08 kg (12.5 lbs)^{footnote 2[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 length.
• 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.