Nearly half of the world’s population experiences labor and delivery, processes which are associated with microscopic or gross tissue injury to the mother. Since this experience is widespread and occurs relatively early in life, the psychosocial, medical, and financial consequences of chronic pain following childbirth could be enormous. Surprisingly, previous studies with long term follow up of new mothers have included pain as a secondary measure and/or have focused on prevalence of pain without determining whether pain predated delivery or even pregnancy itself. This gap in our knowledge is addressed in the current study which focused intentionally on pain which began during the childbirth itself, finding it to be rare in comparison to other physical injuries. Additionally, two central factors hypothesized to confer risk of chronic pain after other injuries including surgery, pre-dating chronic pain and degree of tissue and nerve injury, contributed minimally to the acute and sub-acute pain after childbirth. These two observations, low incidence of chronic pain and minimal effect of degree of tissue injury and history of chronic pain on sub-acute pain, point towards a potential protective effect of pregnancy or delivery on the response to physical injury.
Tissue injury does not inevitably result in chronic pain, yet there are several reasons to anticipate a high incidence of chronic pain from the tissue injury associated with childbirth. Sensory afferents to the uterine cervix and lower uterine segment sprout preceding labor and delivery, become spontaneously active, and become more sensitive to distension of the uterine cervix.17
This may reflect increased estrogen signaling which occurs just prior to the onset of labor, since estrogen increases spontaneous and pressure-evoked responses of uterine cervical afferents by a mechanism involving transient receptor vanilloid-1 channels.18,19
Cervical remodeling, which also begins just prior to the onset of labor, is associated with and dependent upon release of inflammatory substances, including prostaglandins and cytokines, into the cervical tissue,20
and these inflammatory substances are known to sensitize nociceptors.21
One would anticipate that local inflammation and sensory nerve sprouting and sensitization would increase the likelihood of chronic pain by tissue injury in this area. As regards cesarean delivery, traction on ilio-inguinal and ilio-hypogastric nerves and surgical injury to the lower uterine segment with sensitized afferents would also logically increase the likelihood of chronic pain.
In contrast to these considerations, our study suggests that chronic pain after childbirth is much rarer than would be expected from the degree of physical trauma. For example, the incidence of chronic pain after cesarean delivery in our study is over an order of magnitude less than that observed after total abdominal hysterectomy or inguinal herniorrhaphy,10
surgical procedures with similar or lesser degree of trauma to the abdominal wall. Previous studies have suggested a higher incidence of pain after childbirth, but these included pain of uncertain relevancy to the labor and delivery process itself, including predating back pain and headache.5,22,23
We utilized a scripted telephone interview method with simultaneous computer entry rather than interviewing these new mothers in person, and it is conceivable, but unlikely, that some women would have reported pain in person but not over the telephone. Additionally, some women were lost to follow up, and we have no direct knowledge of their incidence of ongoing pain from delivery. For this reason we applied analytic approaches to estimate the likely upper and lower bounds for incidence of pain. Using conservative approaches, the incidence of pain in these estimates still remain remarkably lower than those of abdominal hysterectomy or inguinal herniorraphy.
This is not to say that chronic pain never occurs as a result of childbirth or that its residua are trivial. We previously reported that pain and postpartum depression were present in nearly 10% of women 2 months after cesarean or vaginal delivery, associated with interference with activities of daily living,9
similar to an incidence of 7% 6 weeks after delivery in another large prospective study.23
Although pain was rare in our study 6 and 12 months after delivery, those with chronic pain experienced moderate to severe intensity of pain which was associated with a high utilization of health care and interference with activities of daily living.
We previously showed that pain at 2 months after delivery was strongly predicted by severity of acute pain within 36 hr after delivery,9
but the small incidence of chronic pain after delivery in the longer follow up precluded us from determining a predictive model. We did, however, probe factors which increased the likelihood of acute post-delivery pain and whether the resultant predictive model applied to pain 2 months after delivery, after removing the previously described effect of severity of acute pain. We defined a priori from the dozens of observations those which would likely be related to degree of physical trauma and those related to history of ongoing chronic pain. Although presumed degree of tissue injury contributed to severity of acute pain after delivery, its association was surprisingly small, perhaps reflecting the inability of this somewhat crude approach to quantify tissue injury or a relatively small variability in tissue injury with childbirth. Similarly, the small contribution of pre-dating chronic pain on severity of acute pain and pain 2 months after surgery might reflect a small proportion of women with this condition or the crude assessment of the questions posed. Nonetheless, other studies have utilized these simple methods to demonstrate a clear effect of tissue injury and chronic pain on these outcome measures.3,4,24
Taken together, these observations suggest that either the physical trauma of childbirth, including cesarean delivery, is inadequate to produce chronic pain, or that there are protective biologic or psychosocial factors during pregnancy or the puerperium which nearly eliminate injury-induced chronic pain. Ongoing studies at the Pain Mechanisms Laboratory at Wake Forest University School of Medicine suggest a protective effect of the pueperium from surgical nerve injury induced hypersensitivity,25
consistent with the latter explanation.
In summary, in a prospective study using telephone interviews we observed an incidence of pain 1 year after childbirth of < 1%. Factors presumed to reflect the degree of tissue trauma at delivery and history of chronic pain did not increase the risk of pain 2 months after delivery, after controlling for the effect of severity of acute pain, although they were associated with postpartum depression. These data suggest that chronic pain from childbirth itself does not represent a major public health problem in the United States and we speculate that the puerperium is associated with factors which diminish the risk of chronic pain after physical injury including surgery.