Pathophysiology and Effects of Pregnancy on the Pelvic Floor

Vaginal birth is one of the most important contributors to the development of pelvic floor disorders (PFDs), which include pelvic organ prolapse, urinary incontinence, and anal incontinence. Although PFDs have a collective name, they are separate entities and have distinct causes. Therefore, our ability to prevent birth-related pelvic floor trauma and future PFDs depends on the precise understanding of the mechanisms of injury and how these injuries relate to each specific pelvic floor disorder. Vaginal childbirth places great mechanical stresses on the pelvic floor and would not be possible unless major tissue changes occurred in preparation for birth. These changes include an increase in levator hiatus area and a reduction in connective tissue stiffness. According to biomechanical studies, portions of the levator ani muscles are stretched to 3–4 times their original fiber length to accommodate the fetal head. Such elongation is followed by a period of recovery in most women, where changes in tissue appearance can be clearly seen on radiologic imaging. Muscle recovery results in resumption of the near-normal position in most women over the first 6 months, although it does not always return to a nulliparous state. However, some women sustain unrecoverable levator ani injuries during childbirth that result in pelvic floor dysfunction. For example, pelvic organ prolapse is highly related to levator ani muscle injury and failure of the hiatal closure mechanism, as well as to injuries to the perineal membrane and body resulting from muscle loss and connective tissue disruption; these birth-related injuries can lie latent for many years. While pregnancy itself does not seem to affect urethral function, muscle and connective tissue injuries during delivery often result in worse urethral support, which contributes to the incidence of stress urinary incontinence postpartum. Anal incontinence is likely linked to childbirth by direct injury to the anal sphincter complex and/or damage to the pudendal nerve. Obstetric anal sphincter injury is three- to four-fold more common in primiparous women and is often associated with levator ani avulsion. In this chapter, we will describe the pathophysiologic effects of pregnancy and childbirth on the pelvic floor and elucidate the link between these effects and pelvic floor disorders.