Interpretation
We observed increased straightening of the coccyx within the pregnant
subgroups that had previously delivered solely via CD. Thus, the
potential confounding influence of vaginal birth-related injury was
absent. This suggests that additional remodeling is achieved in
subsequent pregnancies and that these shape differences are likely due
to mechanical and hormonal changes of pregnancy alone, and not delivery.
This is supported by previous work showing that subsequent pregnancies
yielded an increased cellular and hormonal response15.
While this result was not significant and may be better delineated with
an increased sample size and longitudinal data in a future study, it
nevertheless supports the major finding of this study: the coccyx is
undergoing significant changes in shape during pregnancy either due to
remodeling of the coccyx itself, the tissues attached to it, or both.
Though it has been previously noted that the coccyx moves during vaginal
delivery due to direct interaction with the fetal
head7,16, this study found that the coccyx also moves
in pregnancy by rotating posteriorly about the sacrum presumably in
preparation for delivery. It is possible that this straightening
reflects relaxation of the levator ani, anococcygeal raphe, and
coccygeus (which all insert on this structure); remodeling of the
connective tissues supporting the sacrococcygeal joint; or a combination
of these events. Straightening of the sacrum-coccyx may be a maternal
adaptation to lessen the severity of stretch induced injury that has
been demonstrated in prior simulations of vaginal delivery and imaging
studies following vaginal delivery17,18. The effects
of the growing fetus resulting in increased intraabdominal pressure
would likely exhibit a similar effect and could be an alternative
contributing mechanism.
Previous studies describe levator ani defects in parous women and there
was concern about such injuries impacting the results of this
study19–21<sup>21</sup>(21).While
some pregnant women in our study may have delivered abdominally after
entering the second stage of labor, it is important to note that our
pregnant group was vaginally nulliparous and CD in any stage of labor
has been found to be protective of levator injury and the development of
pelvic floor disorders22,23. Thus, the impact of
levator damage was minimized. We did not observe any differences
comparing nulliparous and parous women, which would suggest that either
no injuries were present in our parous cohort or that the coccyx is not
a good indicator of injury.
Previous research by our group evaluating midsagittal pelvic floor shape
found that the levator plate was more posterior, or “relaxed”, in
pregnant women compared to nulliparous and parous women, coinciding with
the posterior motion of the tip of the coccyx resulting in rotation
about the sacrum noted in this study24. While it is
not clear whether remodeling of the coccyx allows for the noted
relaxation or vice versa, the combined implications are that these
maternal changes are necessary and more favorable for vaginal delivery.
Women that fail to remodel sufficiently may be at greater risk for a
complicated delivery and/or maternal injury. Additionally, the failure
to recover the pre-pregnancy shape of the coccyx could be an indication
of injury either to the coccyx or the muscles that insert on it. While
there are currently no clear indications of impaired levator muscle
function in the women analyzed in this study, further research may
provide indication of impaired levator function based on the shape
and/or orientation of the coccyx. Finally, this work indicates that
changes in the levator ani/plate may not be solely related to muscle
injury. If changes in the coccyx persist long-term, this could alter the
length-tension relationships and normal physiologic function of muscles
that insert onto it and potentially compromise support to the pelvic
organs. While much of these discussion points will require more
evidence, this study highlights that the coccyx may be playing a more
important role than has been previously considered.
These findings also have implications for computational models of
vaginal delivery. We have shown that there is significant remodeling and
motion of the coccyx about the sacrum during
pregnancy7. If a model of vaginal delivery did not
account for these changes (i.e. using nulliparous anatomy) then the
geometry may not accurately predict outcomes of vaginal delivery. By not
accounting for the remodeling of the coccyx or pelvic floor, that model
is likely simulating a pelvis that is not fully prepared for vaginal
delivery, meaning the coccyx and levator plate would need to move
further posteriorly to reach the same final configuration as a simulated
pregnant pelvis.