Structural characteristics of term human fetal membranes prior to labour: identification of an area of altered morphology overlying the cervix.

Premature rupture of fetal membranes can have serious clinical implications, especially for the initiation of preterm labour and its consequences. To account for this phenomenon many studies have attempted to identify membrane features that may be uniquely associated with the site of rupture. Our previous work has identified an area of the fetal membrane, following spontaneous term birth which exhibits alterations consistent with structural weakness. The aim of this study was to determine if these changes existed prior to labour. In formalin-fixed paraffin-embedded tissue sections an area of the fetal membrane overlying the cervix, termed the 'cervical membranes', was characterized by an increased thickness of the connective tissue layer (215% increase, P < 0.01) and decreased thickness of both the cytotrophoblast (36% reduction, P < 0.01) and decidual layers (64% reduction, P < 0.01) compared to the rest of the membrane. This resulted in the cervical membranes being significantly thinner (P < 0.05) than the rest of the membrane. Similar changes were also detected in frozen sections of fetal membranes. These regional differences have two important implications in that: (i) the cervical membrane may represent a region of structural weakness susceptible to rupture during labour, and (ii) the paracrine relationships between fetal membranes and the myometrium may be qualitatively affected within different regions of the uterus.