Results
While there was a significant difference between the groups’ ages
(p=0.018), the MANCOVA analysis showed that the observed shape
differences were due to the groups (p<0.001) and not due to
age (p=0.711). Among parous women, the median parity was 2 and median
time since last delivery was 2 years. Overall, it was observed, both
visually and quantitatively, that the sacrum and coccyx were straighter
in pregnant women (Table 1). This was reflected by significant
differences between the three groups in the coccygeal curvature index
(p=0.001), sacrococcygeal curvature index (p=0.002), and sacrococcygeal
angle (p=0.016) (Table 1). All other measures failed to achieve
significance.
Gravidity resulted in a 13.3% increase in the coccygeal curvature index
(89.2 ± 10.0) compared to nulliparous women (78.7 ± 6.6, p=0.003).
Parous women (80.0 ± 5.5) demonstrated a near return to nulliparous
values with a significant decrease of 10.3% relative to the pregnant
group (p=0.004; Figure 2a) and no difference from the nulliparous group.
The additional analysis of subgroups of pregnant women demonstrated an
interesting trend reflecting increased straightening with increased
parity (Figure 2d). However, a similar trend was not observed in the
parous subgroups, suggesting that most of the observed changes are
limited to pregnancy.
The sacrococcygeal curvature index, which includes the combined
curvature of the sacrum and coccyx, also demonstrated a significantly
straighter (8%) sacrococcygeal shape for pregnant (79.2 ± 3.7) relative
to nulliparous women (73.3 ± 5.8, p=0.016). For this metric, the parous
group straddled the pregnant and nulliparous groups but, again, was not
significantly different from either (Figure 2b).
Not surprisingly, differences for the sacrococcygeal angle followed
those of the sacrococcygeal curvature index with the angle for the
nulliparous group at 92.8° ± 10.9° versus 109.3° ± 9.4° for the pregnant
group (p=0.002) and the parous group straddling both but not different
from the nulliparous group (Figure 2c). Both metrics exhibited a similar
trend as that shown for the coccygeal curvature index when evaluating
the subgroups within the pregnant and parous groups individually (Figure
2e/f). It should be noted that none of the measures describing the shape
of the sacrum alone were found to be significant (Table 1). Thus, it is
likely that the significant metrics for the sacrum-coccyx are being
driven by the rotation/elongation of the coccyx.