Which of the following factors might indicate preterm labor?

Preterm labor is characterized by specific signs and symptoms that indicate the onset of labor before the 37th week of gestation. Regular contractions, as indicated in the correct choice, are a key sign of labor. These contractions are typically consistent, occurring at regular intervals, and may lead to changes in the cervix. The presence of pelvic pressure further supports the diagnosis of preterm labor, as this sensation can indicate that the baby is moving down into the pelvis in preparation for delivery.

In this context, sharp, constant pain in the ribs is not commonly associated with labor, as it may indicate other issues, such as muscular strain or respiratory problems. Irregular contractions and lower back pain could be symptoms of Braxton Hicks contractions, which are practice contractions that do not indicate true labor. Random mood swings, while they can occur during pregnancy, are not a definitive sign of preterm labor and do not relate to the physical symptoms that accompany the onset of labor. Therefore, the choice highlighting regular contractions and pelvic pressure accurately reflects the typical indicators of preterm labor, making it the correct answer.