At the time of vaginal delivery, the fetal head will pass through the vagina. The front and back walls of the vagina are pushed apart and at the same time, the lateral attachments are pulled away from their normal line of fixation along the pelvic side-walls (arcus tendinious fascia pelvis). According to the relative sizes of the fetal head and the pelvic opening, there will be some degree of disruption of the normal anatomical structures. The most common anatomical support defect that follows the act of childbirth is the paravaginal defect, which is a detachment of the vagina from its normal lateral attachment to the pelvis. The bladder and urethra lie passively supported on top of the fibromuscular layer of the anterior vaginal wall (pubocervical fascia), just as a person might lie on a trampoline. The rigid frame of the trampoline is the bony ring of the pelvic bones, the springs are the lateral attachments to the pelvic side-walls, and the canvas sheet is the pubocervical fascia that is stretched from side to side and from the front (pubis) to the back (cervix).
When the lateral attachments are pulled away, it is as if the springs of the trampoline are no longer attached to the frame and the canvas will sag downwards. As the canvas comes down, the bladder base and urethra will come down also. The bladder and urethra are passive, resting on the trampoline, but as the canvas descends so also will the bladder base and urethra. It is this, the most common pattern of anatomical support defects, that will lead to problems of stress urinary incontinence and pelvic organ prolapse.
Data suggests that a first vaginal delivery increases the risk of prolapse by three or four-fold and the risk of urinary incontinence by up to five-fold. This risk goes up with each additional vaginal delivery.
Certain American obstetricians have been advocating for performing more Cesarean sections. They argue that a C-section is almost as safe as a vaginal birth; that it eliminates pelvic floor damage and is safer for the infant. They also add that many women want the baby delivered by C-section. Therefore, the prophylactic use of elective C-section to prevent pelvic organ prolapse and urinary incontinence is gaining increased attention.
However, according to recent research, an elective C-section does not seem to eliminate the risk of prolapse and urinary incontinence. Research suggests that women undergoing elective C-section have a two or three-fold higher risk of pelvic organ prolapse and incontinence when compared with women who are delivering for the first time.
C-section also causes more maternal morbidity and mortality than does vaginal delivery. In the short term, a C-section doubles or triples the risk for infections, hemorrhage and hysterectomy. It also increases the risk of thromboembolism by two to five-fold and causes surgical injury in about 1 percent of cases. Babies delivered by C-section have a higher risk of lung disorders and operative lacerations.
So, if we assume that vaginal delivery is responsible for 85 percent of all cases of prolapse and urinary incontinence, and that women have an 11 percent lifetime risk of undergoing surgery to treat these conditions, we would have to do 23 C-sections to prevent one such surgery later in life. We would also have to assume that C-section is completely protective, something we are not sure is true.
Therefore, instead of offering elective C-section in an attempt to prevent future prolapse or incontinence, doctors should evaluate what can be done in the management of vaginal deliveries to protect pelvic floor function.