BACKGROUND Despite impressive progress in the science and technology of reproduction, the mechanism by which labour is initiated in humans remains obscure. OBJECTIVE As the labour in humans is a distinct event differs from what happens in animals, this study aims to gather the current theories that could explain when and why the onset of human parturition occurs. METHODS In a comprehensive review study done at the School of Medicine and Dentistry, University of Bristol, United Kingdom, MetaLib, the university web-based electronic library, was cross-searched for the factors behind the onset of labour in humans through different medical databases such as; Allied and Complementary Medicine Database (AMED), BIOSIS Previews on Web of Knowledge, PubMed, Cochrane Library, Medline and Web of Science, in-addition to the relevant printed medical journals and periodicals. RESULTS The study revealed that among the potential factors involved in the process of human parturition are the changes in hormonal levels of estrogen and progesterone, increased production of prostaglandins and oxytocin, and the high levels of corticotrophin releasing hormone and cortisol are some among the potential factors involved in the process of human parturition. Inflammatory reactions with the release of cytokines are among the most accepted theories for term and preterm labours. It is most likely that the interaction between all these factors and others, yet to be discovered, play in harmony to initiate the process of labour in women. CONCLUSIONS The result show that birth is a result of complex, partially defined, events that are tightly regulated by a variety of mechanisms and mediators of endocrine, nervous and immune systems. Unfortunately, none of them is completely elucidated.