The principal risk factor predictive of preterm birth is:

Choice A rationale

Magnesium sulfate is primarily used for neuroprotection in preterm infants and to prevent seizures in severe preeclampsia, not as a first-line tocolytic for preterm labor.

Choice B rationale

Morphine sulfate is an opioid analgesic used for pain management, not for inhibiting preterm labor contractions.

Choice C rationale

Terbutaline is a beta-agonist used as a tocolytic to relax uterine muscles and delay preterm labor. It helps decrease the intensity and frequency of contractions, making it suitable for managing preterm labor.

Choice D rationale

Betamethasone is a corticosteroid used to accelerate fetal lung maturity in preterm labor, not for direct contraction inhibition.

Choice A rationale

Retained tissue can cause postpartum hemorrhage, but with a firm uterus and no other signs of retained placenta, this is less likely the cause here.

Choice B rationale

Trauma is the most likely cause of increased bleeding in this scenario. The prolonged oxytocin induction and macrosomic infant suggest a higher risk of lacerations or uterine atony despite the firm uterus.

Choice C rationale

Thrombin disorders cause bleeding due to clotting issues. However, this patient shows signs of active bleeding and clotting, making this less likely.

Choice D rationale

Uterine atony, indicated by a soft, boggy uterus, is a common cause of postpartum hemorrhage, but in this case, the uterus is firm, so it's less likely to be the cause.