Symmetry in Paraspinal Muscles as a Predictor of the Development of Pregnancy‐Related Low Back and Pelvic Pain: A Prospective Study

OBJECTIVE: To determine the asymmetry in the paraspinal muscle before pregnancy and evaluate its association with pregnancy‐associated lumbopelvic pain (LPP). METHODS: This was a prospective case–control study conducted from January 2017 and December 2018. A total of 171 subjects (mean age ± SD, 27.4 ± 5.8 years) were finally divided into the LBP group, PGP group, and no LPP group. Each subject was asked to follow a standardized clinical imaging protocol before the pregnancy. The area of muscles (multifidus, erector spinae, and psoas muscles) on the axial slice at mid‐disc of L(4)–L(5) and L(5)–S(1) were segmented and then the cross‐sectional area (CSA) of a particular muscle was measured by outlining the innermost fascial border surrounding each muscle. The mean value of F‐CSA's ratio to T‐CSA (F/T CSA) was used to determine whether the bilateral paraspinal muscle was asymmetrical. Total muscle CSA (T‐CSA) represents the sum of CSA of interested three muscles. The signal intensity can distinguish fat and muscle tissue in a different range. Based on this, functional CSA (F‐CSA), represented by fat‐free area, was evaluated quantitively by excluding the signal of the deposits of intramuscular fat. Total muscle CSA (T‐CSA), functional CSA (F‐CSA), and the ratio of F‐CSA to T‐CSA (F/T CSA) were measured unilaterally and compared between groups. Logistic regression was performed to determine the risk factors for pregnancy‐associated LPP. The Pearson correlation coefficient was performed to test the relationship between asymmetry in F/T‐CSA and pain rating. RESULTS: A total of 124 subjects (72.5%) (28.5 ± 5.2 years) had LPP during pregnancy. Forty‐eight (38.7%) individuals had low back pain (LBP) and 76 (61.3%) had pelvic girdle pain (PGP). Seventy‐six women (44.4%) were determined to have asymmetry in paraspinal muscle according to the definition in this methods section. The duration of follow‐up was 24 months postpartum. A total of 39 (31.5%) women unrecovered from LPP. F/T‐CSA was significantly decreased for LBP in the PGP group than in the and control group (0.03 ± 0.02 vs 0.05 ± 0.03 vs 0.12 ± 0.05, P < 0.001). Meanwhile, significant differences were detected in both groups (all P < 0.001). In patients with LBP, the level of paraspinal asymmetry, represented by the difference in F/T‐CSA, was positively correlated with pain scores (r = 0.52, P < 0.01). However, no statistically significant correlation between pain scores and paraspinal asymmetry was found in PGP (r = 0.42, P > 0.05). Asymmetry in the paraspinal muscle (adjusted OR = 1.5), LBP (adjusted OR = 1.6), LPP in a previous pregnancy (adjusted OR = 1.4), sick leave ≥90 days (adjusted OR = 1.2), and heavy labor (adjusted OR = 1.2) were risk factors for the unrecovered LPP during pregnancy. CONCLUSIONS: Asymmetrical muscular compositions could lead to abnormal biomechanics for the segmental motions. Lateral‐directed physical training and stretching may help decrease the occurrence and severity of this condition.