Final answer:
Retained placental fragments are the most common cause of late postpartum hemorrhage.
Step-by-step explanation:
The nurse would identify retained placental fragments as the most common cause of late postpartum hemorrhage. When the placenta is not completely expelled from the uterus after childbirth, it can cause bleeding. It is important for the obstetrician to examine the expelled placenta and fetal membranes to ensure they are intact and there are no fragments left behind.