Final answer:
The primiparous woman preparing for an emergency cesarean birth for fetal distress is at the greatest risk for early postpartum hemorrhage due to complications associated with emergency deliveries and potential underlying issues.
Step-by-step explanation:
Among the listed scenarios, the woman at greatest risk for early postpartum hemorrhage (PPH) is a primiparous woman (G 2 P 1 0 0 1) being prepared for an emergency cesarean birth for fetal distress. This is because an emergency cesarean delivery is a major surgical procedure that can lead to increased blood loss and uterine atony, both risk factors for PPH. Moreover, the situation of fetal distress may indicate underlying issues such as prolonged labor or infection, which could further complicate the postpartum period.
In contrast, although a woman with preeclampsia on magnesium sulfate has an increased risk due to the potential for uterine atony, this condition is managed closely to reduce risks. A multiparous woman with an 8-hour labor or a primigravida in spontaneous labor with preterm twins could also potentially be at risk, but they do not present with the immediate complications associated with emergency cesarean delivery under distress.