Objective The shape of the sacrum-coccyx was defined and compared in nulliparous, pregnant, and parous women to provide insight into anatomical adaptations that afford vaginal delivery. Design A retrospective study comparing midsagittal anatomical measurements based on MRI of the sacrum and coccyx from 63 subjects. Setting Magee-Womens Research Institute and Northshore University HealthSystem. Population 23 nulliparous, 14 pregnant, and 26 parous women who had an MRI taken that included the necessary bony anatomy. Methods Twelve measurements were taken on scans between the ages of 20 and 49 that had a pelvic MRI scan with or without contrast were analyzed. Subjects were categorized based on parity and gravidity. Main Outcome Measures Length, angles, and curvature indices describing midsagittal sacrum and coccyx shape Results Overall pregnant women had a significantly straighter and more posteriorly oriented coccyx when compared to nulliparous women. This was reflected by a change in 3 measures at the univariate level. The coccygeal curvature index was higher in pregnant (89.2  10.0) women relative to nulliparous (78.7  6.6, p=0.003) and parous (80.0  5.5, p=0.004) women. The sacrococcygeal curvature index and sacrococcygeal angle also increased in the pregnant as compared to the nulliparous group (73.3  5.8 versus 79.2  3.7, p=0.016; 92.8  10.9 versus 109.3  9.4, p=0.002, respectively) with no difference between pregnant and parous groups for these measurements. Conclusions Pregnancy-induced posterior motion of the coccyx, which allowed for the combined sacrum-coccyx shape to straighten, effectively widens the obstetric outlet for vaginal delivery.