Figure 5. Concept of the initiation of parturition in cattle (according to Shenavai et al., 2012). Prepartal endocrine changes similar to the sheep have been confirmed in the prepartal cow (see Fig. 4 and 5). However, different from the late pregnant sheep with the placenta as the only relevant source of progesterone, in cattle the corpus luteum is the predominant source of progesterone throughout gestation and the prepartal decline in maternal progesterone level is clearly associated with luteolysis.