Bacteria activate cell-mediated immunological responses, leading to the production and release of cytokines, including interleukin-1 (IL-1), IL-6, tumor necrosis factor-α (TNF-α), prostaglandins, or endotoxins such as lipopolysaccharides, which may precipitate pre-term labour if they reach the foeto-placental unit. Endotoxin or lipopolysaccharide of Gram-negative bacteria may be detected in biological fluids of sterile compartments such as the amniotic cavity. Administration of endotoxin to pregnant animals results in embryo resorption, pre-term labour, and foetal death. A higher risk of spontaneous pre-term delivery has been associated with genetically driven excessive amniotic fluid IL-Iβ or with a disturbance of bioavailability and/or bio-response of this cytokine, which is central to the pro-inflammatory reaction to infectious stimulants. The foetus also has a role in pre-term birth; the foetus recognizes a hostile intrauterine environment and may precipitate labour by premature activation of the foetal-placental parturition pathway.