Case-referent study of cystic ovaries as a risk indicator for twin calvings in dairy cows

The association between cystic ovaries and twin births in dairy cows was analyzed in an epidemiologic study. The case-referent study design was chosen. The cases were all twin-bearing cows during the observation period. Two different groups of cows giving birth to single calves were used as controls. One group consisted of cows from the same herds as the twin-bearing cows; the second group consisted of cows randomly selected from the entire population. The frequency of cystic ovaries was significantly higher in control cows from herds with twin-bearing cows than in control cows randomly selected from the general population. This finding suggests an indirect association between the occurrence of cystic ovaries and the birth of twins. The direct association between cystic ovaries and birth of twins was evaluated by odds ratio. The magnitude of this measure varied according to breed and the control group used for calculation. A biological explanation for this finding could be that the frequency of intrauterine death and resorption of one twin fetus differs among breeds and cows with or without a history of cystic ovarian disease.