Effect of thickness on the fracture characteristics of fetal membranes

The mechanical characteristics of the chorioamniotic membrane were evaluated with a new burst test apparatus by rupturing 35 specimens that were taken from a sample of seven afterbirths. ‘Strength’, ‘stiffness’, ‘toughness’, and ‘ductility’ were measured. Mechanical characteristics did not change significantly with variation in thickness. While ductility should not correlate with thickness, the lack of a significant increase in strength, stiffness and toughness with an increase in thickness is most unusual and requires an explanation. Subsequently, an additional experiment, which was designed to ascertain the mechanical stability of membrane specimens with prolonged exposure to air, showed a dramatic increase in stiffness as membranes were allowed to dry. The increase in stiffness indicates that strength and toughness also increase with drying, provided that ductility remains constant. Thus, the degree of hydration of the membrane, which is reflected in thickness, regulates mechanical characteristics. The increase in the water content of certain amniotic layers has a lubricating effect on the amnion-chorion interface. This lubrication increases as term approaches. Thus, hydration must increase as gestation progresses, and strength, stiffness, and toughness must decrease until the membrane is weak enough to rupture at the end of gestation. Thus, hydration must increase as gestation progresses, and strength, stiffness, and toughness must decrease until the membrane is weak enough to rupture at the end of gestation.