“Mommy, how will the baby get out of your tummy? Will it hurt you?”

Small children recognize the impossibility of birth. This was also the conclusion of one of our bright biomechanical engineering graduate students about 15 years ago who was tasked with making a biomechanical model simulating birth. After reviewing the literature about pelvic floor aperture diameter and fetal head size, he opined that birth is biomechanically impossible. Anyone watching a vaginal birth for the first time would have agreed until they were amazed by what occurs. How can the pelvic floor possibly dilate enough for a full-term baby to emerge? Is anyone surprised that some women are permanently damaged in the process?

What is surprising is that this remarkable phenomenon escaped scientific study for so long. Sadly, up to 1 of 5 women have pelvic floor disorders so severe that they require surgery during their lifetime for injuries largely attributable to changes resulting from birth.^1,2 Ignorance of the nature of birth-induced injury and of the mechanisms responsible for eventual pelvic organ prolapse has blocked efforts at targeted, individualized, and effective prevention. Of the 3 million women who deliver vaginally each year, >200,000 will develop pelvic organ prolapse later in life, the pelvic floor disorder most strongly associated with birth. Even with gold-standard operations, and in the best of hands, the anatomical failure rate following prolapse surgery is 25%.³ Information to improve prevention and treatment are therefore urgently needed. Vaginal delivery is the single most important risk factor for prolapse. Two births confer an 8-fold increased risk for needing surgery later in life.^4,5 At the heart of the issue is this question: what pelvic floor injury occurs during vaginal birth that results in 10-20% of women needing surgery later in life, and how and why does it happen?

In their article, “In vivo evidence of significant levator ani muscle stretch on MR images of a live childbirth,” Sindhwani and colleagues⁶ have added important new information about the birth process. Using the remarkable magnetic resonance images of a birth published by Bamberg and colleagues,⁷ they performed an innovative and detailed biomechanical analysis of the geometric changes that take place in the pelvic floor of a woman having her second birth. Previous work in this regard used prepregnancy geometry and average measurements of fetal size to assess the biomechanical changes needed during birth, with resultant stretch ratios (the ratio of muscle length as the fetal head is pushed out to the original muscle length) of 3.3⁸ and 3.1.⁹ What has been missing are human data collected during birth, so it was not known how these theoretical estimates would correspond to actual measurements.

Through careful analysis, Sindhwani and colleagues⁶ found a stretch ratio of 2.5 (248%) in their analysis of a delivery. Is this value consistent with or different than prior theoretical values? This value is 22% lower than the 3.2 average of previous estimates. This lower value is probably attributable to 2 factors. First, this baby was 19% smaller than average (2585 g compared to the average birthweight of 3389 g¹⁰). In addition, this baby was born to a mother who had previously given birth vaginally. After a spontaneous vaginal delivery, the levator hiatus area is, on average, 17% larger (15.2 cm² after first vaginal delivery vs 13.0 cm² after cesarean delivery)¹¹; this would mean the levator ani would stretch less to encircle the same-sized baby.

When these factors are considered, this article supports the previous theoretical estimates of pubococcygeal stretch ratios of about 3 from 2 independent groups. It is important that this has been done by actually measuring a human birth. Because of the many assumptions that must be made in modeling, the study findings published in this issue are absolutely critical for validating other estimates.

Why should we care about this? For millennia, women have paid a lifelong price for their unique role in vaginal delivery, and we as obstetricians and gynecologists are responsible for their care. Given the remarkable scientific tools at our disposal, what is our generation doing to create a safer way for women to give birth? In the last 2 decades, important advances have been made in understanding risk factors for birth-related pelvic floor injury and how pelvic floor injury translates to pelvic floor disorders. Investigators in the fields of biomechanics, imaging, cell biology, and epidemiology have joined obstetrician gynecologists to make a serious investigation of this remarkable phenomenon and its after-math. Progress in understanding birth and injury is now accelerating.

Unfortunately, most of the identified risk factors for pelvic floor injury are not known until after birth. Since cesarean birth carries both immediate and long-term risk to the mother that is probably higher than the consequences of vaginal birth, universal cesarean delivery is not an answer. If the injury rate is 10%, you would have to do 9 cesarean deliveries to prevent 1 pelvic floor operation later in life, and this does not take into account the proportion of pelvic floor disorders not caused by childbirth–an illogical equation. However, if one could identify that 1 person, or maybe individuals with a 50% risk for future pelvic floor disorders, then the risk-benefit ratio of targeted cesarean delivery becomes more favorable.

Why is the stretch ratio clinically important? It is an essential number in understanding injury. Women (eg, the one in the published study) with a large hiatus and small baby would not benefit from injury prevention efforts and should be reassured and spared the potential harms of intervention. Women with a predicted large stretch ratio, on the other hand, might benefit from this information in considering whether to have a cesarean delivery. This concept has been demonstrated in a prospective study by Rostaminia and colleagues¹² comparing the levator ani stretch ratio at 36 weeks’ gestation on ultrasound with subsequent severe levator ani trauma, and in a biomechanical analysis published by Tracy and colleagues¹³ predicting the percentile of maternal capacity likely to deliver various-sized fetal heads without levator ani trauma. Preventing levator ani trauma is important not only because these injuries do not usually heal, but also because they increase the risk for future pelvic organ prolapse and other poor outcomes.^14,15 Measuring levator hiatus and fetal head size late in pregnancy could provide important predictive information that would allow for maximizing benefit while minimizing unintended harm and is a concept that is ready for clinical trials.

In a 2004 presidential address published in this journal with the optimism of youth,¹⁶ this editorialist proposed a goal for 2025 in our field, which was to have 25% improvement in prevention and 25% improvement in the treatment of pelvic organ prolapse. Thirteen years have passed, with 7 to go. Great strides have been made in pelvic floor science, but they have not yet resulted in measurable improvements in prevention and treatment. On the other hand, as the current study clearly demonstrates, we are gaining the necessary quantitative scientific framework to design rational interventions that have the possibility of spurring progress. As obstetricians and gynecologists who have the responsibility for helping women with both birth injuries and pelvic floor disorders, the obligation to solve this problem rests firmly on our shoulders. Although we have not yet achieved our goal, it is now clearly in sight.