Final answer:
Oxytocin is the uterine relaxant used for retained placenta, stimulating contractions to aid in its expulsion.
Step-by-step explanation:
The uterine relaxant used for retained placenta is oxytocin. Oxytocin, which is also called Pitocin when synthesized artificially, stimulates uterine contractions that aid in the expulsion of the fetus and placenta from the uterus during labor. At the end of pregnancy, the synthesis of oxytocin receptors in the uterus increases, and oxytocin released from the posterior pituitary gland assists with the contraction of the smooth muscle cells of the uterus. These contractions are vital for the last stage of labor, which includes the passage of the placenta after the baby's birth. If labor halts or if there is a need to assist with the expulsion of a retained placenta, synthetic oxytocin can be administered to enhance uterine contractions, facilitating the removal of the placenta from the uterine wall.