The placenta, which is of fetal origin, has over 100 highly vascular spots called cotyledons which interconnect with similar highly vascular spots on the uterus called caruncles. Together, these form a unit called the placentome. Retained fetal membranes occur when the cotyledons fail to separate from the caruncles. Fetal membranes are normally expelled within 3-8 hours after birth, and they become classed as 'retained' if they are still present 12 hours or more after birth. Duration of retention is highly dependent on uterine muscular contractions, which cease after 36 hours post-partum. Expulsion of the placenta after 36 hours post-partum is primarily dependent on bacterial putrefaction.
Retention of fetal membranes is more common in dairy cows than in beef cows and there are several factors that predispose a cow to retained fetal membranes, including:
- Male calves, due to their larger size
- Dystocia or difficult birth
- Short or long gestation
- Uterine torsion
- Induction of labour using steroids (dexamethasone)
- Early calving (retards separation)
- Fetal membranes are usually obviously hanging from the vulva
- Transient reduction in appetite and milk yield
- Bad-smelling discharge from the vulva
- Signs of secondary or systemic infection, including pyrexia, inappetance, depression etc.
The membranes usually spontaneously pass within 4-10 days as the caruncles undergo necrosis and bacterial putrefaction, without the need for treatment. Manual extraction is only recommended if the placenta will come freely as forcible removal will cause small lesions to the uterus. Administration of intrauterine oxytetracycline will reduce the bacterial load present in the uterus and may prevent development of endometritis or metritis. However if intrauterine antibiotics are administered while the placenta is still retained, then it will prolong the time to expulsion as it will prevent bacterial digestion of the placetome. Systemic antibiotics should be considered when animals are pyrexic (body temperature >39.5 degC) as this is an indication of systemic illness such as with metritis. Intravenous calcium may help if the retention is due to hypocalcaemia and is adminstered soon after parturition.
Prognosis is good and there is generally no long term effect on maternal reproductive potential, particularly if there are no secondary infections or problems arising due to the retained membranes. However, retention of the placenta will delay uterine involution and predispose to development of either endometritis, which can lead to subfertility and increase the calving to conception interval, or metritis, which can cause serious systemic illness and carries a guarded to poor prognosis.