Innovative research has led to marked improvement in medical knowledge. Advances in technology have allowed us to attain a clearer understanding of many physiologic processes. Despite these impressive gains in knowledge, the cause of the initiation of the process of human labor remains unclear. Potential factors include changes in hormonal levels of estrogen and progesterone, increased production of prostaglandins, and elevation of levels of corticotropin-releasing hormone, as well as increased sensitivity of the myometrium to endogenous oxytocin. It is most likely that interactions between these factors play an essential role in the process of labor, but the precise mechanism of onset still eludes us.