Biomechanical properties of the human umbilical cord.

The umbilical cord is a complex and fascinating structure that connects the fetus to the placenta and encases the umbilical vessels. The response of its tissues to mechanical loading due to fetal movements and uterine contractions is not well understood. The aim of this study is the evaluation of the mechanical properties of the main components of the human umbilical cord. Fresh umbilical cord specimens were collected from neonates born at term of the gestation and submitted to compliance tests. Furthermore, uniaxial tensile and stress-relaxation tests were performed on samples of umbilical vein and Wharton's jelly. Both materials exhibited nonlinear stress-strain response with increasing strain, increasing the elastic modulus (E(high) about 10-20 times E(low)) and significant viscoelastic behavior. In addition, anisotropy of the vein was observed. Although the circumferential properties of the vein (mean E(high) about 2.4 MPa) were similar to those after birth, the longitudinal stiffness of both materials was higher (mean E(high) over 10 MPa) and comparable to that of the ligaments. These findings suggest a mechanism of protection acting against excessive elongations of the cord, which could cause undue restriction of the umbilical vessel area and interference with the fetal blood circulation.