Main Findings
The midsagittal sacrum-coccyx measures differed across nulliparous, pregnant, and parous groups with the sacrum-coccyx assuming a straighter shape during pregnancy as the coccyx rotates posteriorly about the sacrococcygeal joint. Although not significantly different from the nulliparous group, parous values tended to straddle the nulliparous and pregnant groups, suggesting that the sacrum-coccyx shape of some parous women does not completely return to the nulliparous range even years postpartum.
Interestingly the most significant statistic was the coccygeal curvature index, while the least significant was the coccygeal angle. At first glance, they appear to describe the same shape changes, but the curvature index is much more sensitive to subtle changes. Figure 3 demonstrates how the curved length (simplified as 2 lines) can remain the same while the coccygeal angle and straight length increase. The increase in the simplified straight length, and by extension the curvature index, (4.3%) is larger than the increase in angle (1.7%). This suggests that when evaluating coccyx shape the curvature index may highlight differences that the coccygeal angle would not capture.