Final answer:
The most common causes for subinvolution of the uterus are retained placental fragments and infection, which disrupt the normal involution process leading to complications like postpartum hemorrhage.
Step-by-step explanation:
Retained placental fragments and infection are the most common causes of subinvolution of the uterus. After childbirth, the uterus undergoes involution to return to its pre-pregnancy size; however, when there are retained fragments of the placenta or infection, this process can be disrupted, leading to subinvolution. An obstetrician must examine the expelled placenta and membranes to ensure completeness, as any remnants can lead to postpartum hemorrhage.
These retained pieces, along with infection, prevent the normal contraction and shrinkage of the uterus. Breastfeeding helps facilitate uterine involution by promoting contractions. In severe cases, if the placenta does not expel within a reasonable timeframe or if remnants are detected, medical or surgical intervention might be necessary.