Association Between Self-Perceived Physical Fitness Assessed Using the International Fitness Scale (IFIS) and Pregnancy-Related Low Back Pain in Physically Active Pregnant Women

Background and Objectives: Pregnancy-related low back pain (PLBP) is one of the most frequent musculoskeletal complications during pregnancy and may negatively affect physical function, occupational performance, and quality of life. Although regular physical activity (PA) is recommended during pregnancy, the relationship between self-perceived physical fitness (SPPF) and PLBP remains insufficiently explored. The aim of this study was to analyze the association between SPPF assessed using the International Fitness Scale (IFIS) and PLBP in a cohort of physically active pregnant women. Materials and Methods: A secondary analysis of a prospective cohort study including 147 physically active pregnant women was performed. All participants fulfilled the World Health Organization recommendations for PA during pregnancy (≥600 MET·min/week). PA was assessed using the International Physical Activity Questionnaire (IPAQ), whereas SPPF was evaluated using the IFIS. PLBP was assessed using structured questionnaires and the Visual Analog Scale (VAS). Multivariable logistic regression analyses were performed to evaluate independent associations between IFIS scores and PLBP. Results: PLBP was reported by 95 women (64.6%). During pregnancy, women without PLBP showed significantly higher IFIS scores across all evaluated fitness dimensions compared with women with PLBP (all p < 0.05). Women with PLBP additionally exhibited significant reductions in perceived speed/agility and flexibility compared with prepregnancy values. In multivariable analyses, higher global IFIS scores during pregnancy were independently associated with lower odds of PLBP (adjusted OR: 0.30; 95% CI: 0.15–0.60; p < 0.001). No significant associations were observed between IFIS scores and occupational interruption, umbilical artery pH, or instrumental delivery. Conclusions: Lower SPPF assessed using the IFIS was associated with PLBP in physically active pregnant women. Higher IFIS scores during pregnancy were independently associated with lower odds of PLBP, highlighting the potential clinical relevance of SPPF beyond PA volume alone. Given its simplicity, low cost, and ease of administration, the IFIS may constitute a practical tool for the assessment of SPPF during routine prenatal care. However, prospective studies are needed to clarify the temporal relationship between SPPF and PLBP and to determine its potential predictive value.