What is the primary concern when a 25-year-old woman presents with lower back pain and pelvic heaviness in late pregnancy?

The primary concern when a 25-year-old woman presents with lower back pain and pelvic heaviness in late pregnancy is preterm labor. These symptoms can be indicative of the body's preparations for labor, especially if they are accompanied by other signs such as contractions or changes in vaginal discharge.

Preterm labor is characterized by regular contractions that result in changes to the cervix before 37 weeks of gestation. The presence of lower back pain and a feeling of pelvic heaviness can signal that the body is beginning to enter the labor process prematurely. It is crucial to address this promptly, as preterm labor can pose risks to both the mother and the developing fetus.

While the onset of labor is a part of the differential diagnosis and is indeed a concern as the pregnancy progresses, in the context provided, where the woman is presenting with specific symptoms that may signal preterm labor, it becomes the primary concern to evaluate and manage accordingly. Sepsis and fetal distress are serious conditions but are less likely to present with the specific symptoms described in the question.