Final answer:
The most common causes of subinvolution in postpartum women are retained placental fragments and infection.
Step-by-step explanation:
The most common causes of subinvolution in postpartum women are retained placental fragments and infection. It is important for the obstetrician to examine the expelled placenta and fetal membranes to ensure that they are intact. If fragments of the placenta remain in the uterus, they can lead to postpartum hemorrhage and hinder the process of involution.