The nature of structural defects associated with velamentous and marginal insertion of the umbilical cord.

Previous studies have documented an increased incidence of structural defects in association with velamentous and marginal insertion of the umbilical cord. Evaluation of 4,677 consecutive placentas at the University of California (San Diego) Medical Center revealed 454 marginal insertions and 72 velamentous insertions. An increased incidence of structural defects was found only in association with a velamentous cord insertion. Delineation of the structural defects according to their developmental pathogenesis revealed that the majority were due to deformation of a normally formed part. These data and the finding of a disproportionate number of twins with a velamentous insertion suggest that competition for space at the implantation site leads to both the velamentous insertion of the cord and the associated structural defects.