Here's what the Merck manual says about it:
Cows: Retention of fetal membranes, or retained placenta, usually is defined as failure to expel fetal membranes within 24 hr after parturition. Normally, expulsion occurs within 3-8 hr after calf delivery. The incidence in healthy dairy cows is 5-15%, while the incidence in beef cows is lower. The incidence during lactation is increased by abortion, dystocia, hypocalcemia, twin birth, high environmental temperature, advancing age of the cow, premature birth or induction of parturition, placentitis, and nutritional disturbances.
The precise pathogenesis of retained fetal membranes is poorly understood, but normal maturation and loosening of the placenta begin during late pregnancy and are marked by alterations in the collagen of the placentome. During parturition, changes in uterine pressure, reduction in blood flow, and physical flattening of the placentome during uterine contractions contribute to final loosening and expulsion of the fetal membranes.
Diagnosis is usually straightforward as degenerating, discolored, ultimately fetid membranes are seen hanging from the vulva >24 hr after parturition. Occasionally, the retained membranes may remain within the uterus and not be readily apparent, in which case their presence may be signalled by a foul-smelling discharge. In most cases, there are no signs of systemic illness. When systemic signs are observed, they are related to toxemia. Uncomplicated retention of fetal membranes is unsightly and inconvenient for animal handlers and milkers but generally not directly harmful to the cow. However, cows with retained fetal membranes are at increased risk of developing metritis, ketosis, mastitis, and even abortion in a subsequent pregnancy. Cows that have once had retained fetal membranes are at increased risk of recurrence at a subsequent parturition.
Manual removal of the retained membranes is no longer recommended and is potentially harmful. Trimming of excess tissue that is objectionable to animal handlers and contributes to gross contamination of the genital tract is permissible. Untreated cows expel the membranes in 2-11 days. Routine use of intrauterine antimicrobials has not been found to be beneficial and may be detrimental. Although advocated at various times, oxytocin, estradiol, prostaglandin F2α, and oral calcium preparations have not been shown to hasten expulsion of retained membranes. When systemic signs of illness are present, systemic treatment with antimicrobials and NSAID is indicated. In herds in which incidence of retained fetal membranes is unacceptably high, predisposing causes should be sought and eliminated.