Dr. Elliot Berlin is an award winning prenatal chiropractor, childbirth educator and labor doula.  His Informed Pregnancy® Project aims to utilize multiple forms of media to compile and deliver unbiased information about pregnancy and childbirth to empower new and expectant parents to make informed choices regarding their pregnancy and parenting journey.  


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The Pelvic Paradox 

By Elliot Berlin I DC

During third-trimester prenatal visits, many well-intentioned practitioners place a great deal of emphasis on measuring the unborn baby, relaying information and subliminal expectations to the expectant mother. As a result of this practice, many women have become fearful that their baby is too big and that their pelvis is too small to safely attempt vaginal delivery. Some women are told outright that if their baby measures over 8.5 pounds, they would be better off scheduling an elective Cesarean or being induced early to prevent their infant from growing “too big” prior to birth. It is an absolute fact that only a small number of babies and mothers will sustain injuries from a “trial of labor” (attempt to deliver vaginally). However, the overwhelming majority of babies, including those that measure over 9 pounds, do very well with vaginal delivery, even if one of the shoulders gets stuck temporarily on the way out—the most common complication of delivering a baby that is measuring large. In practice, we often see large babies that measure over 9 or 10 pounds born to small mothers with no complications. We also see smaller babies who sometimes “get stuck” navigating their way out of a larger pelvis. I refer to this common occurrence as the Pelvic Paradox—the mystery of how a seemingly large baby can safely and smoothly birth through a small pelvis, and how smaller babies can fail to descend through a larger pelvis or become injured on the way out. There are several explanations for the Pelvic Paradox, and this article addresses three: prediction of fetal size, physiology, and state of mind. 


Estimation of Fetal Weight. Practitioners typically use two methods to estimate the weight of an unborn baby during prenatal visits. Unfortunately, neither method is particularly accurate. The first measure involves the doctor feeling with his/her hands to examine the mother’s abdomen, trying to estimate—based on clinical experience—how big or small the baby is. 


The second method employs an ultrasound, which comes up with an estimated fetal weight by using a series of formulas and calculations based on fetal shapes and sizes. Although ultrasound technology has evolved dramatically in the last few decades—you can even see your child’s face in 3D while it is still in the womb!— it is still an unreliable way to attempt to measure a baby’s weight. The margin of error for an ultrasound is plus or minus 1 or more pounds. You could be told your baby is on track to weigh 9 pounds, and you should schedule a Cesarean, but your baby may actually weigh less than 8 pounds at full term!  


Physiology Anatomy. refers to the structure or parts of the body, and Physiology refers to how those parts function or work together. Even if highly accurate measurements of the mother’s pelvis and baby’s size were possible (and they are not), it would still be difficult to predict in advance those who will easily deliver vaginally and those who will not. This is partially because these measurements fail to take into account physiology. Can a basketball fit through a rubber band? The answer is not solely based on the size of the basketball or the rubber band, but also on how stretchy the rubber band is, and how inflated or rigid the basketball is. During childbirth, the bones of the baby’s head are normally able to mold, alter their shape, and overlap to narrow its overall diameter, allowing the baby to pass through a pelvic opening smaller in diameter than that of the baby’s head. Additionally, the mother’s pelvis is not a single solid bone, but formed by several bones connected by moveable cartilage, ligaments, and joints.


When fully functional, the mother’s pelvis has the dynamic ability to expand, contract, and help accommodate the baby’s movements, not to mention encourage safe passage of the baby through the birth canal. The natural ability of the mother’s pelvis to stretch and the normal compressibility of the baby’s head and body helps explain how a larger fetal head, when functioning properly, can emerge safely through a smaller pelvic opening that is also functioning as it should. During pregnancy, I encourage expectant mothers to take steps to maximize the normal function of their pelvis with modalities such as yoga, massage, and chiropractic care. 


When it comes to measuring the size of a woman’s pelvis (pelvimetry) to determine labor outcomes, a research paper by the American Academy of Family Physicians (AAFP) found that “Current practice is to allow all women a trial of labor regardless of pelvimetry results. This makes the routine performance and recording of clinical pelvimetry a waste of time, a potential liability, and an unnecessary discomfort for patients.”


State of Mind. Placing fear in the mind of a mother prior to and during labor can significantly slow or completely halt the natural progression of labor and delivery. Mothers who are told that their babies are “large” and “may get stuck” may be more likely to experience fear, making it harder to relax into the ideal frame of mind—and body—during labor. Women who are encouraged in their choices regarding natural labor and delivery and who feel safe in their environment, confident in their practitioner, and supported by other people around them, may have a substantially better progresion through labor and, overall, much better outcomes. 


When it comes to determining whether or not to schedule a Cesarean section or to induce a baby early because a physician believes the baby might be on the larger side, the AAFP found that “Elective Cesarean section is seldom a suitable alternative (to vaginal delivery), and elective induction of labor appears to increase rather than decrease the Cesarean section rate.” The most commonly cited concern regarding vaginal delivery of larger babies is injury to a group of nerves in the neck called the brachial plexus. Permanent brachial plexus injury can leave portions of the arm and/or hand weak or paralyzed. In practice, however, damage to the brachial plexus is extremely rare. A recent analysis estimated that in order to prevent just one case of permanent brachial plexus injury, 3,700 women carrying babies with an estimated fetal weight of 9.9 pounds or more would need to have an elective Cesarean. Of course, Cesarean deliveries come with their own set of risks for the mother, the baby, and future pregnancies and deliveries. Everyone wants to do all they can to ensure the safest delivery of a healthy baby. While no means of delivery is risk free, the vast majority of vaginal deliveries occur without significant complications. Take steps to become informed and empowered during your pregnancy. Choose a practitioner you trust to help you make informed choices and support those choices and to provide you with the best environment for a safe delivery without unnecessary fear.