Pregnancy results in considerable compensations in biomechanics and structure of the spine. During pregnancy the thoracic (mid-back), lumbar (low back), and whole pelvic structure undergo changes in preparation for the subsequent delivery. Loosening of the usually firm joints under hormonal influence leads to changes in posture, weight bearing, degree of spinal curvature, and dynamics of vertebral motor units.
The change in posture occurring with pregnancy is a universal observation in all chiropractic offices. That some of the change in posture remains permanent if not corrected after delivery is also a common clinical observation. The mechanism of this postural change is not obvious: whether it is due to persistent abdominal laxity or to primary changes of the joints of pelvis and spine has not been determined.
At the onset of pregnancy, postural changes are subtle. As pregnancy proceeds to term, the increased size of uterus and fetus causes the occurrence of a backward compensation. The first area to compensate is the lumbosacral junction, With a greater angle of pelvic inclination, a greater curvature of the lumbar spine results. With the resultant increase in lumbar curve, the thoracic spine compensates in the opposite direction. These compensations in the mid-back open the facet joints resulting in instability.
While these movements at the sacroiliac joints are small, they are quite definite. By the tenth to twelfth week of pregnancy a hormone, relaxin, is produced by the corpus luteum. Under this hormone?s influence, the ligaments that hold the sacoliac joints and the pubic symphysis in place soften and stretch. The articulations between these joints widen and become more moveable, accentuating the movements described above. The purpose of the changes is to increase the size of the pelvic cavity and to make delivery easier. This relaxation is progressive and becomes maximal by the beginning of the third trimester and continues for up to three months after pregnancy owing to the action of the hormone relaxin.
Besides softening the ligaments at the symphysis pubis and sacroiliac joints, similar changes occur in the ligaments supporting structures of the lower spine, including the outer fibers of the lower intervertebral discs. This relaxation, plus the sagging abdominal wall, leads to strain on the ligaments supporting the lumbo-sacral area. The relaxation of the abdominal muscles also leads to over-action of the spinal muscle group, thus accentuating the lumbar curve further and altering the whole line of static force.
This instability and looseness of the joints due to relaxin may cause discomfort in the pubic and lumbar region. Due to this instability and pain, a pregnant woman will attempt to keep joints “locked” during locomotion and will assume the characteristic duck-waddling walk of late pregnancy.
However, even with the “locking” of the joints, there is still the disturbing feature of increased shearing stress applied to the lumbosacral area produced by the increased lumbar curve. The shift in weight from front to back increases the pressure on the back of the lumbar vertebrae jamming the posterior facet joints. This can manifest painful joint capsule irritation, bony impingement, and nerve root irritation not to mention the associated progression of permanent degenerative changes if the abnormal biomechanics are not corrected.
The posterior weight-bearing of pregnancy also produces lumbar disc changes. The anterior disc has a high tensile stress and there is an increased constant compression stress on the posterior of the disc and an increased intradiscal pressure. This can also be aggravated by water retention during pregnancy and may lead to increased pressure within the disc and to protrusion of the intervertebral disc. Disc protrusion can be a source of cord or nerve irritation and pain that accompanies pregnancy.
As can be seen, full-term pregnancy is a risk factor for low back disc injury due to increased load on the disc structures imposed by the increased weight of the uterus and its contents coupled with the hormonal influence. A study by O’Connell reveals that of the 347 patients who had given birth to one or more children, thirty-nine percent of the women developed symptoms of disc protrusion either during pregnancy or postpartum. A later study by Kelsey suggests that the causative factor of disc injury is related to pregnancy rather than to the care of the children after pregnancy. Consequently the biomechanical stress of pregnancy may be a more important consideration in disc injury than the lifting of children, especially in subsequent pregnancies.
After the birth, improvement in low back dysfunction and structure is generally seen and attributed to the passage of the mechanical burden imposed by the fetus and recovery of the fibrous structures to normal tensile strength. Several months may elapse before these fibrous structures regain their original tensile strength and abdominal exercise must be implemented for normality to be reattained.
As has been stated, the biomechanical changes and stress to the spine and pelvis are quite significant during pregnancy. Chiropractic evaluation and care during this time of high spinal stress is highly recommended. Following the birth, rehabilitative exercise for weakened spinal and abdominal muscles along with chiropractic spinal care to assist the proper restoration of normal spinal biomechanics is advised. Who better to monitor spinal changes during and after pregnancy than a specialist in spinal biomechanics, your family chiropractic?