In vitro umbilical cord wrapping and torsion: possible cause of umbilical blood flow occlusion.

OBJECTIVE: Intrapartum fetal heart rate decelerations and bradycardia are often attributed to umbilical cord occlusion without knowing the anatomic basis of that occlusion. We hypothesized that umbilical cord twisting while looped around fetal parts could occlude blood flow.

METHODS: Using an in vitro preparation, human umbilical cord veins were perfused at one end with water at approximately 40 cm H2O. The cords were looped around pipes that approximated the diameter of fetal body or limb parts, after which the perfused segment of cord was twisted until water flow stopped. The number of rotations needed to stop perfusion was recorded for each length of twisted cord (4, 6 and 8 cm) and for each pipe diameter.

RESULTS: There were 21 completed studies. All cords demonstrated that a decreasing number of twists were needed to stop venous flow as the segment twisted became shorter (from 8 to 4 cm). For each segment length, the number of twists required to stop flow decreased as the pipe diameter narrowed.

CONCLUSION: This model demonstrates that a wrapped umbilical cord, particularly with a short segment between the placental insertion and the fetal body part, may be predisposed to cord occlusion in response to fetal rotation.