A 34-year-old pregnant woman at 30 weeks gestation reports lower back discomfort, especially when standing. She denies injury, and the pain improves with rest. Which assessment finding best explains her symptoms?

During pregnancy, the enlarging uterus displaces the center of gravity anteriorly, necessitating a compensatory lumbar lordosis. This postural shift increases the mechanical strain on the paraspinal muscles and lower intervertebral discs. Hormonal changes, specifically the release of relaxin, further contribute to ligamentous laxity and joint instability. A. Increased lumbar lordosis from the shift in the center of gravity: As the fetal weight increases, the mother leans backward to maintain balance, exaggerating the inward curve of the lower spine. This compensatory posture causes chronic muscle tension and lower back pain. It is the most common musculoskeletal finding in the third trimester. B. Decreased mobility of the sacroiliac joints: Pregnancy actually causes an increase in sacroiliac joint mobility due to the hormone relaxin. This increased laxity is necessary for the widening of the birth canal during delivery. Decreased mobility is not a typical physiological finding in pregnancy-related back pain. C. Increased thoracic kyphosis due to breast enlargement: While breast enlargement can slightly alter upper body posture, it does not typically cause the primary lower back discomfort reported by the patient. Kyphosis affects the upper back (hunchback appearance), whereas the patient's symptoms are localized to the lower back. D. Muscle atrophy of the paraspinal muscles from reduced activity: Most pregnant women experience muscle strain from the increased load rather than atrophy. While activity may be reduced, the body must work harder to support the extra weight. Atrophy would involve a loss of muscle mass, which is not a standard finding in healthy pregnancy.