Pregnancy-Related Pelvic Organ Displacement: A Case-Control Magnetic Resonance Imaging Study Using Threshold-Defined Classification

Background: The impact of pregnancy on the pelvic floor is not yet fully understood. This study aimed to investigate pelvic organ displacement during pregnancy and identify its key influencing factors. Methods: This retrospective case-control study analyzed 238 pregnant women (gestational age range: 12–41 weeks) and 238 age-matched (±1 year) non-pregnant controls. All participants underwent pelvic magnetic resonance imaging (MRI) at two hospitals during the same hospitalization period. Distances from the bladder neck (BN), cervix (C), and posterior fornix (PF) to the pubococcygeal line (PCL) were measured using MRI. These distances were compared between pregnant and non-pregnant groups. Pregnancy-related “inferior” or “superior” positions were classified using thresholds derived from the non-pregnant group (mean ± 1.96 standard deviations [SD]). Univariate and multivariate logistic regression analyses were performed to identify factors independently associated with positional deviations in the pregnant group. Results: The mean gestational age of the pregnant group was 32.3 ± 5.8 weeks. Compared with controls, pregnant women exhibited inferior BN (19.2 ± 6.1 vs. 23.3 ± 4.3 mm, p < 0.001) and C positions (19.1 ± 7.8 vs. 21.6 ± 5.3 mm, p < 0.001), but superior PF positions (42.3 ± 10.5 vs. 31.7 ± 8.1 mm, p < 0.001). In univariate analyses, the fetal engagement depth was significantly associated with positional changes in all pelvic organs (p < 0.001 for all). After adjusting for confounders, a fetal engagement depth of ≥40 mm was independently associated with increased odds of inferior BN (odds ratio [OR] = 5.04, 95% confidence interval [CI]: 1.59–16.01, p = 0.006) and C (OR = 11.46, 95% CI 2.53–51.92, p = 0.002) positions, whereas a depth of <20 mm predicted superior PF positioning (OR = 12.24, 95% CI 4.82–31.11, p < 0.001). Additionally, the presence of a low-lying placenta (OR = 2.96, 95% CI 1.18–7.39, p = 0.020) or placenta previa (OR = 2.70, 95% CI 1.29–5.64, p = 0.008) was significantly associated with superior PF position. Conclusions: This study demonstrates that pregnant women exhibit significant positional alterations in pelvic organ anatomy. Notably, fetal engagement depth emerged as a robust biomechanical correlate of these anatomical changes.