Extracellular matrix homeostasis is a key process in the maintenance of the tensile strength of the amniochorion. This tensile strength guarantees the role of the membranes as a physical and functional boundary for the fetus during human pregnancy. Pathological rupture of these structures before 37 completed weeks of gestation is known as preterm prelabour rupture of the membranes (PPROM) and it is a major cause of spontaneous preterm labour and preterm birth. A mechanism involving the activation of matrix metalloproteinases (MMP)-9, a 92-kDa type IV collagenase, as an essential mediator of tissue damage is under investigation. The proposed mechanism involves the abnormal expression and activity of MMP-9 with subsequent connective tissue degradation taking place at a time that does not synchronise with other events of labour. The local physiological signal by amniochorion cells to induce MMP-9 expression is not known, but bacterial products and/or the proinflammatory cytokines, IL-1beta and TNF-alpha, as paracrine or autocrine signals may trigger these processes in pregnancies complicated with intra-amniotic infection. These signalling pathways indicate complex cooperative and bidirectional communications between amnion and choriodecidua in response to bacterial products, which include intermembranous cytokine traffic and signalling between tissues. Products secreted in culture by amniochorion and choriodecidual leucocytes, obtained from women who delivered following normal labour in the absence of infection, condition a specific microenvironment that induces collagen degradation in fetal membranes. Further characterisation of the role of choriodecidual leucocytes in the control of extracellular matrix degradation in amniochorion is currently under way.