Introduction
Hormonal changes during pregnancy cause tissue remodeling, resulting in connective tissue softening to facilitate vaginal delivery1. Throughout the female lifespan, the levator ani muscles and pelvic connective tissues act to close the levator hiatus. During vaginal delivery, the pelvic floor undergoes stretching to allow passage of the fetal head2. Biomechanical and finite element modeling have been used to identify and clarify the amount and location of the maximum muscle and connective tissue stretch during vaginal birth3,4. The majority of the research in this area has concentrated on the pelvic floor musculature, pubic symphysis, and sacroiliac joints1,5,6. However, previous work from our lab demonstrated the potential need for tissue softening at the sacrococcygeal joint to accommodate the fetal head. During simulations of vaginal delivery, mechanical loads introduced as the fetal head pushed the tip of the coccyx posteriorly, forcing the muscles and connective tissues engaged with the coccyx to stretch7. Posterior motion of the coccyx about the sacrococcygeal joint has also been noted in studies that conducted magnetic resonance imaging (MRI) during vaginal delivery, but has yet to be quantified8.
The sacrum and coccyx were chosen as the focus of this study as they provide an important attachment point for the coccygeus and levator ani. The coccygeus inserts on the superior two vertebrae of the coccyx (Co1 and Co2) and the most inferior vertebrae of the sacrum (S5). The coccygeus is distinct from the levator ani, which contribute to the support of the pelvic organs and help stabilize the coccyx. Among the levator ani, the iliococcygeus and anococcygeal raphe insert on the coccyx. In this way, a homeostatic relationship is likely formed between the coccygeus and the laxity of the sacrococcygeal joint, which would be driven by the connective tissues surrounding the joint and the tension generated by the iliococcygeus and anococcygeal raphe. Therefore, with any increase in pressure (e.g. increasing intrabdominal pressure generated by a growing fetus), change in muscle function, or change in the connective tissue stiffness at the sacrococcygeal joint would ultimately result in a measurable difference in the orientation of the maternal coccyx. Additionally, during delivery, physical interaction between the fetal head and the pelvic floor muscles could also cause a significant alteration in the orientation of the coccyx.
Other studies have shown significant movement of maternal bony pelvic structures during pregnancy, which can result in asymmetrical sacroiliac joints or pain localized to the coccyx5,9,10. While all women sustain stretching of their pelvic floor during birth, only some will experience injury resulting in pelvic pain, pelvic organ prolapse, urinary incontinence, and/or fecal incontinence that can develop immediately or decades after delivery11,12. Improved metrics quantifying the degree of remodeling during pregnancy and following delivery may provide insight into predictors of vaginal birth-related injuries and/or complications and pelvic floor disorders.
The objective of this study was to define changes in the position of the coccyx relative to the sacrum and the midsagittal shape of the sacrum and coccyx in women before, during, and after pregnancy. To do this, we aimed to measure variations in the combined sacrum-coccyx shape induced by pregnancy and delivery by comparing midsagittal lengths, angles, and curvature indices between nulliparous, pregnant, and parous women. We hypothesized that the orientation of the coccyx would be more posterior in pregnant women—providing more room in the pelvis for the fetus during delivery by increasing the anterior-posterior diameter of the obstetric outlet—and then return, but not completely, towards nulliparous values in parous women.