Separation of amnion from choriodecidua is an integral event to the rupture of normal term fetal membranes and constitutes a significant component of the work required

Objective This study was undertaken to determine the sequence of events that occur during fetal membrane (FM) rupture and to compare the biophysical properties of intact FM with its separated individual components (amnion and choriodecidua). Study design FM physical properties were determined with computerized, specially adapted, industrial, strength testing equipment and the rupture sequence (in vitro) video documented. Separated individual FM component properties were compared with those of reapproximated components, and of intact FMs. Results The sequence of FM rupture was (1) FM components stretch together under load; (2) amnion separates from choriodecidua; (3) choriodecidua ruptures; (4) amnion distends further, nonelastically; and (5) amnion ruptures. In all FMs tested, amnion was stronger, stiffer, and more ductile than choriodecidua. The sum of work required to rupture separated FM components (amnion + choriodecidua), or reapproximated components, was significantly less than that of intact FMs. Conclusion Separation of amnion from choriodecidua occurs as part of normal term FM rupture. FMs become significantly weaker as a result of this separation.