Anal sphincter disruption (sometimes referred to as a fourth-degree laceration or laceration from the vagina through the circular muscle that surrounds the anus) is a birth injury that can have serious long-term consequences for the mother, including incontinence, pelvic pain, pain with intercourse, and psychological trauma.  It has long been suspected by some obstetric practitioners, including midwives, that the length of the second stage of labor (the pushing part) may impact on the likelihood of lacerations in the anal sphincter; when it is very short, forcing the baby through the birth canal before it is ready; or very long, stretching out the vaginal and perineal tissues beyond their capacity to recover. Researchers at an obstetric center in Cambridge, U.K. recently tested this theory by examining the records of 4831 first-time mothers who delivered a singleton infant at term (37-42 weeks) in either the high-risk obstetric unit or the low-risk birthing center to look at length of second stage, with and without operative delivery (forceps or vacuum extractor), and sphincter injury.  Three hundred twenty five women were noted to have sustained such an injury.  In women who delivered spontaneously, there was no relationship between the length of labor and the occurrence of a sphincter injury.  Older age of the mother, weight of the baby, and Asian ethnicity were found to be related to an increased risk of this injury in these women.  In women who had instrumental deliveries following a prolonged second stage, there was an increase of injury.  The authors state that “the association between prolonged second stage and sphincter injury for instrumental deliveries is likely explained by the risk posed by the use of the instruments themselves or by delay in initiating instrumental assistance. Attempts to modify the duration of the second stage for prevention of sphincter injuries are unlikely to be beneficial and may be detrimental” (my italics).  In other words, trying to shorten a prolonged second stage (usually defined as over 2 hours) by using forceps or a vacuum extractor is not helpful and may very well be harmful.

 “Influence of the Duration of the Second Stage of Labor on the Likelihood of Obstetric Anal Sphincter Injury.” Catherine E. Aiken, MB/BChir, PhD, Abigail R. Aiken, MB/BChir, MPH, PhD, and
Previous studies have implicated prolonged second stage as an independent, though not a major risk factor for anal sphincter injury. According to the JFP literature review referenced below, major risks are first birth, short perineum, instrumental delivery, epidural anesthesia, episiotomy, birthweight over 4 kg, persistent posterior position of the baby’s head, and a previous anal sphincter tear.  Midline episiotomy (commonly used in the U.S.) is twice as likely as mediolateral episiotomy (more common in Europe) to result in this injury.  A forceps delivery with a midline episiotomy
increases the risk by 25 times the baseline, far more than any single factor.
For more information about anal sphincter injury, the article, “Obstetric Anal Sphincter injury: How to Avoid, How to Repair: A Literature Review”, in the online Journal of Family Practice, is a good overview. http://www.jfponline.com/home/article/obstetric-anal-sphincter-injury-how-to-avoid-how-to-repair-a-literature-review/cd36e99bc32b6b902077c5182033ea9d.html
NS Feb 22, 2015