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The Second Stage of Labor - The Anatomy of Vaginal Births

The human body is a marvelous thing, and the anatomy and physiology of birth is no different. Did you know that it is NOT your pelvic floor that “pushes” the baby out, your uterus does!


Anatomy of the Pelvic Floor

The pelvic floor muscles sit within the pelvis between the tailbone and the pubic bone and provide organ support, promote bladder and bowel control, support the spine and pelvis and optimize sexual and reproductive health. The pelvic floor muscles are composed of three layers of muscles with the deepest layer known collectively as the Levator ani and composed of the pubococcygeus, puborectalis and illeococcygeus. These muscles have a “U” shape with the middle component of the “U” the location of the anal opening, vaginal and urethral opening. This region is collectively known as the urogenital hiatus (outlined above in yellow) and is the area that widens to allow passage of the baby through the pelvic floor musculature during delivery. The normal baseline activity of the levator ani muscles keeps the urogenital hiatus closed against the opening action of intrabdominal pressure that may be produced with activities to include, but not limited to, coughing, sneezing, laughing, and lifting to prevent episodes of urinary and/or fecal incontinence from occurring. (1)


The Stages of Vaginal Birth


Labor and delivery can be divided into three different stages:


Stage 1 – Dilation (opening) of the cervix

Stage 2 – Delivery of the baby

Stage 3 – Delivery of the placenta


In this post we will be focusing primarily on the second stage of labor, delivery of the baby. When attempting a vaginal birth, a woman has successfully transitioned from the first to second stage of labor when she has achieved full cervical dilation (widening of the cervix) and effacement (thinning of the cervix). At time of transition between the first and second stage of labor if the baby is in a cephalic (“head down”) position the baby’s head will be positioned just above the level of the pelvic floor musculature (i.e. the Levator ani). At this stage of the process the woman will now be educated to begin to “push”.


So what specifically happens when you “push” during delivery?


To answer this question, first you must understand the anatomy of the inner core musculature and how the coordination and use of these muscles influence intra-abdominal pressure management and secondarily pressure generation.


You may have either tried this before or seen someone else attempt to do it – The contents of a soda can are non-compressible if you attempt to do so when the can is full and sealed. This is secondary to the amount of pressure within the can, with the external pressure needing to be greater than the internal pressure to successfully crush the can.

This same analogy can be applied to the inner core musculature and the content of the abdominal-pelvic region. As shown in the picture to the left (2), the inner core muscles make up a cylindrical structure similar to the aluminum structure of a soda can with the pelvic floor the ‘base’ of the can and the respiratory diaphragm the ‘top’ of the can.


When you inhale or bear down, your diaphragm contracts and “flattens” thus increasing pressure within the abdominal cavity and causing the pelvic floor muscles to descend or “lengthen”. During the second stage of labor, contractions will often occur every 3 minutes and last for approximately 1 minute with it important to coordinate these contractions of the uterus with bearing down so that as the uterus contracts to “push” the baby out, the pelvic floor muscles relax to increase the width of the Levator hiatus thus allowing for passage of the child through the birth canal. According to our current body of literature, a single volitional “push” timed to coincide with the height of each uterine contraction is a more efficient pushing style than pushing at other times. (3)


Birthing Positions During the Second Stage of Labor

Leading up to labor and delivery the hormone relaxin is released by your ovaries and the placenta to help better prepare the body for birth. This hormone helps loosen and relax muscles, joints and ligaments. During the second stage of labor, the baby is “engaged” within the pelvis with ideally the head engaged at and/or slightly above the level of the pelvic floor, otherwise known as the pelvic outlet (outlined in yellow above).


Your pelvis to composed of 3 primary bones:

  1. Innominate – This is your main “pelvic” bone with your pelvis composed of two innominate bones, left and right.

  2. Sacrum – Your sacrum and coccyx are your “tailbone” and is the connection point between the paired innominate bones at the base of the spine.

The muscles which make up the Levator ani have attachment points along the sacrum, coccyx and innominate bones. With the assistance of relaxin, the joints which connect these bones develop greater laxity allowing them to have greater amounts of mobility to increase the size of the pelvic outlet in preparation for labor and delivery. Research has shown that positions which help promote sacral “flexibility” will significantly help reduce the duration of the second stage of labor – In other words positions that allow the sacrum to rotate forward to increase the diameter of the pelvic outlet. So to summarize in simpler words, these are positions where the direct pressure is taken off of the tailbone such as sidelying, quadruped, and a supported squat. Ultimately though women should choose a position that is most comfortable for them to labor in, something that is not standardized across all women.


How a Pelvic Physical Therapist Can Help Prepare You For Birth


Despite popular belief, a “strong” pelvic floor is not necessary for a successful vaginal birth, in fact pelvic floor overactivity and/or discoordination may extend labor. A well-coordinated inner core, to include the diaphragm, pelvic floor and abdominal muscles, can help improve your ability to successfully increase pressure within the abdominal cavity while relaxing the pelvic floor to make your “pushing” more effective.


A pelvic physical therapist has the training and education in anatomy and physiology to be able to provide you with education and training during your pregnancy so that you may know how to properly coordinate these muscles to better prepare yourself for birth.


If you are currently pregnant and looking to better educate yourself so that you may be better prepared for labor and delivery, it is recommended that you seek consultation from a pelvic physical therapist during the third trimester of your pregnancy. But, if you are experiencing dysfunction during your pregnancy, to include pelvic pain and/or incontinence, it is recommended that you seek consultation prior to the third trimester to optimize your pregnancy experience and allow you to maintain the lifestyle you wish to have throughout your pregnancy.


References:

  1. Ashton-Miller, James A., and John O.L. DeLancey. “On the Biomechanics of Vaginal Birth and Common Sequelae.” Annual Review of Biomedical Engineering, vol. 11, no. 1, 2009, pp. 163–176, https://doi.org/10.1146/annurev-bioeng-061008-124823.

  2. Massery M. Musculoskeletal and neuromuscular interventions: a physical approach to cystic fibrosis. J R Soc Med. 2005;98 Suppl 45(Suppl 45):55-66. PMID: 16025768; PMCID: PMC1308809.

  3. Berta, Marta, et al. “Effect of Maternal Birth Positions on Duration of Second Stage of Labor: Systematic Review and Meta-Analysis.” BMC Pregnancy and Childbirth, vol. 19, no. 1, 2019, https://doi.org/10.1186/s12884-019-2620-0.

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