Focus Article Modeling childbirth: elucidating the mechanisms of labor Xinshan Li, 1∗ Jennifer A. Kruger, 2 Martyn P. Nash 1 and Poul M.F. Nielsen 1 The process of childbirth and the mechanisms of labor have been studied for over a century, beginning with simple measurements of fetal skull and maternal pelvis dimensions. More recently, X-rays, ultrasound, and magnetic resonance imaging have been used to try and quantify the biomechanics of labor. With the development of computational technologies, biomechanical models have emerged as a quantitative analysis tool for modeling childbirth. These methods are well known for their capabilities to analyze function at the organ scale. This review provides an overview of the state-of-the-art finite element models of the mechanics of vaginal delivery, with detailed descriptions of the data sources, modeling frameworks, and results. We also discuss the limitations and improvements required in order for the models to be more accurate and clinically useful. Some of the major challenges include: modeling the complex geometry of the maternal pelvic floor muscles and fetal head motion during the second stage of labor; the lack of experimental data on the pelvic floor structures; and development of methods for clinical validation. To date, models have had limited success in helping clinicians understand possible factors leading to birth-induced pelvic floor muscle injuries and dysfunction. However, much more can be achieved with further development of these quantitative modeling frameworks, such as tools for birth planning and medical education.  2009 John Wiley & Sons, Inc. WIREs Syst Biol Med 2010 2 460–470 T he mechanism of labor is described as a series of passive movements of the fetus through the birth canal. Historically, this has only been explained in relation to the bony pelvis and the fetal skull. However, the importance of the associated soft tissues in this process has recently become more apparent. In particular, the surrounding muscles known collectively as the levator ani (LA), which includes the iliococcygeus, pubococcygeus and puborectalis, play a central role during labor. Labor is divided into three stages: the first stage being the onset of effective uterine contractions until full dilatation of the cervix; the second stage is from full dilatation of the cervix until birth of the baby; and the third stage is from birth of the baby until delivery of the placenta. ∗ Correspondence to: shannon.li@auckland.ac.nz 1 Auckland Bioengineering Institute, The University of Auckland, New Zealand 2 Department of Sport and Exercise Science, The University of Auckland, New Zealand DOI: 10.1002/wsbm.65 Second stage necessitates the passage of the fetus through the maternal pelvis. In order for this to occur, the fetal head has to undergo a series of internal translations and rotations, aided by maternal effort in the form of active pushing and uterine contraction. However, with each contraction, the resistance from the LA pushes back on the fetal head (or presenting part), 1,2 which may influence the progress of labor depending on the biometry and function of the pelvic floor. 3,4 It is also during the second stage of labor that birth-induced injury to the LA is likely to occur. Muscle loss and denervation are frequently observed in magnetic resonance imaging (MRI), ultrasound and electromyography studies of post-vaginal delivery. 5–8 Failure to recover from such trauma may lead to permanent neuromuscular damage and pelvic floor dysfunction. 8–11 This is also an important factor in the development of pelvic floor prolapse in parous women, 12–14 which is a common medical condition requiring substantial surgical costs each year. 15,16 A better understanding of the nature and cause of pelvic floor prolapse is desirable for reducing the medical costs and improving women’s health and lifestyle. 460  2009 John Wiley & Sons, Inc. Volume 2, July/August 2010