Final answer:
The nurse should be most concerned about subinvolution if retained placental fragments are present, as these can lead to postpartum hemorrhage and impede the uterus returning to its pre-pregnancy size.
Step-by-step explanation:
The postpartum nurse should be most concerned with the presence of subinvolution if there are retained placental fragments from delivery. Subinvolution refers to the inadequate shrinking of the uterus to its pre-pregnancy size. Retained placental fragments can hinder this process and potentially lead to complications such as postpartum hemorrhage. It is critical for an obstetrician to examine the placenta to ensure no parts are left behind because they can interfere with the involution process and contribute to excessive bleeding.