Childbirth is often associated with physical injury. Whether women deliver vaginally or through cesarean section, many experience unavoidable physical injury and may be at risk for the development of chronic pain. Researchers wanted to determine whether childbirth represents a major cause of chronic pain in women.

In the first study, 1,228 women were interviewed within 36 hours of delivery. Of these women, 76 percent successfully completed a telephone interview at two months, and, if they had pain, at six and 12 months after delivery.

Pain that began at the time of delivery was remarkably rare six and 12 months later. Only 1.8% of the women who had pain that began at the time of delivery had pain 6 months later and only 0.3% of women with pain beginning at the time of delivery had pain at 12 months.

"The study suggests there may be a protecting mechanism that is active around the time of childbirth to prevent chronic pain from physical injury," said James C. Eisenach, M.D., Professor, Anesthesiology-Obstetric and Gynecologic Anesthesia, Wake Forest Baptist Medical Center. "In our accompanying lab study, we analyzed the sources of this natural protection in hopes we may be able to develop treatments to prevent chronic pain from happening after other types of trauma or surgery, similar to vaccines to prevent infectious diseases."

In the second study, preliminary research in rats was conducted to assess a possible mechanism for a low incidence of chronic pain after childbirth. The effect of pregnancy and delivery on hypersensitivity induced by peripheral nerve injury (spinal nerve ligation (SNL)) to mechanical stimuli was investigated. The effects of spinal injection of atosiban (an antagonist of oxytocin, a hormone that is important during and after childbirth) and naloxone (an opioid receptor blocker) were also examined, as well as oxytocin concentration in lumbar cerebrospinal fluid.

Results showed pain-like behavior from SNL surgery recovered more rapidly when the surgery was performed shortly after delivery compared to non-pregnant control rats. The more rapid recovery was temporarily blocked by spinal injection of the oxytocin antagonist, atosiban, but not the opioid antagonist, naloxone, and cerebrospinal fluid oxytocin was increased in rats after delivery. The more rapid recovery was also blocked if the pups were removed (weaned) from the mother, which would likely reduce maternal oxytocin release. These results suggest elevated oxytocin concentrations in the brain and spinal cord in the postpartum period protects mothers against chronic hypersensitivity from peripheral nerve injury.

"Oxytocin in the brain is considered important to mother-baby bonding, trust, love and social engagement. These results suggest the surge in oxytocin around childbirth may also speed recovery from the pain caused by childbirth," continued Dr. Eisenach.

"Both studies provide important new information toward understanding the development of chronic pain after childbirth," added Cynthia A. Wong, M.D., Northwestern University, Chicago, in an accompanying editorial. "Understanding whether and how pregnancy protects against the development of post-traumatic chronic pain is not only important to women who give birth and their children, but may also provide therapeutic targets for future prevention and treatment of chronic pain in other populations."

Dr. Eisenach is the Editor-in-Chief of Anesthesiology.

For more information, visit the Anesthesiology website at anesthesiology.org.