Normally, the membranes are ruptured by the foal over the avillous cervical star region of the chorioallantois. The chorion (red velvety surface) and allantois (shiny surface containing many blood vessels) should be examined. Normally, the chorionic surface color ranges from red to brownish red. Patches of discolored, thick, exudate-covered chorioallantois at the cervical star or between the two horns may indicate an ascending or focal placentitis, respectively. The fetal membranes should be examined for completeness, paying particular attention to the presence of the edematous gravid horn tip and puckered nongravid horn tip.
The amnion has a white translucent appearance and may contain many blood vessels near the umbilical cord. Small, pale amniotic plaques can normally be seen along the umbilical cord.
Fetal and neonatal foal deaths may be associated with pathologic changes present in the fetal membranes. Typically, the fetal membranes weigh ~10%–11% of the foal’s body weight. Placentitis or placental edema may increase the weight of the membranes. Integrity of the junction between the fetal and maternal components of the placenta is essential for normal fetal development. The mare has a diffuse microcotyledonary, epitheliochorial, nondeciduate type of placentation, which directly reflects the presence of abnormalities in the endometrium.
Fetal membranes that are not expelled within 3 hr of parturition are considered to be retained. Fetal membrane retention may be complete, but commonly only the nongravid horn is retained. If the typically puckered nongravid tip is not observed, it is assumed to be retained. If any membranes are hanging from the vulva of a postpartum mare, the amnion and cord should not be cut and removed, because their weight provides tension thought to enhance placental separation and expulsion. Fetal membranes not passed within 3–10 hr are considered to be pathologic and can lead to metritis, endotoxemia, and subsequently laminitis with fatal results. Accordingly, it is prudent to treat the condition as potentially serious. If dystocia or traumatic uterine manipulation has occurred, aggressive treatment for retained membranes should be instituted immediately after parturition.
For early (3–8 hr) retention, 10–20 IU oxytocin can be repeatedly administered IV or IM every 30 min until the fetal membranes have passed. The dose of oxytocin should be decreased if the mare shows severe signs of colic or discomfort. Milking or sucking also stimulates endogenous release of oxytocin. Mares that retain fetal membranes >8 hr should be administered broad-spectrum antibiotics, such as potassium penicillin (22,000–44,000 IU/kg, IV, qid), gentamicin (2.2 mg/kg, IV, qid), or flunixin meglumine (0.25–0.5 mg/kg, IV, tid).