Monitoring cervical changes.

This is because cervical changes indicate the progress of labor and the risk of preterm delivery.

Preterm labor is defined as regular uterine contractions with cervical dilation and effacement before 37 weeks of gestation.

The nurse should assess the cervical length, dilation, effacement, and position frequently to determine the need for interventions to stop or delay labor.

Choice A is wrong because monitoring vital signs is not specific to fetal well-being.

Vital signs can reflect maternal health, infection, or complications, but they do not directly measure fetal status.

Choice C is wrong because monitoring fluid intake and output is not specific to fetal well-being.

Fluid balance can affect maternal hydration, electrolytes, and blood pressure, but it does not directly measure fetal status.

Choice D is wrong because monitoring maternal preference is not specific to fetal well-being.

Maternal preference can affect the comfort, satisfaction, and coping of the mother, but it does not directly measure fetal status.