It is not uncommon for some women to be told during pregnancy something along the lines of “your baby is measuring big and you may end up with a cesarean.” Other times, a multiparous mother may be concerned her pelvis is too small because she’s had a prior c-section or instrumental birth. Statements and experiences like these can be extremely detrimental to a woman’s confidence and emotional health as she prepares for birth. Creating a scenario of possible cephalopelvic disproportion (CPD), where the fetal head is too large to successfully navigate the dimensions of the maternal pelvis, can lead to a self-fulfilling mindset that sabotages labor before it even begins. Despite the fact that true CPD is widely believed to be rare, this condition is commonly expressed prenatally as a plausible concern that could stymie a natural, vaginal birth (https://americanpregnancy.org/healthy-pregnancy/labor-and-birth/cephalopelvic-disproportion/). During labor, it is cited as the main cause of “failure to progress,” the number one reason for cesareans. With this caveat front of mind, a mother may begin to doubt her body, its ability to successfully maneuver through the landscape of labor and birth, and its potential to grow a baby that is the right size for her body. In the end, she may doubt herself and her ability to birth her own child.
HOW DOES A PROVIDER KNOW IF I HAVE CPD?
The short answer is they really don’t, and the quote from Anne Frye in the third picture above gives a good explanation as to why. Hormones are working throughout the course of pregnancy to prepare a woman’s body for the processes of nurturing a rapidly-growing baby, birthing it, and subsequently nourishing it through nursing. Relaxin, the hormone responsible for softening and “relaxing” the ligaments and connective tissues of the pelvis and reproductive organs, doesn’t exert its full effect until near the end of pregnancy. Because of this, the only way to truly know whether a woman can birth a baby of any size is to allow the process to take place. The combination of pelvic ligament relaxation, cervical softening and dilation, ever-lengthening and strengthening contractions, and molding of the fetal head all work in concert to allow the female pelvis to accommodate an amazing variety of fetal sizes and presentations.
Often, the preliminary “diagnosis” of CPD is made with little or no concrete evidence that the condition exists. There are true cases (about 1 in 250) where pelvic size, shape, or gross abnormality secondary to trauma or developmental defect can make the prospect of vaginal birth more unlikely or even unsafe, especially when compounded with a less-typical fetal presentation. But these circumstances are uncommon and often have to be readily apparent as no single diagnostic tool has proven sufficient on its own to confirm them. Ultrasound, for example, is often the scapegoat used to introduce the possibility of CPD, especially later in pregnancy when the information it yields is more inaccurate than those performed in the first trimester. According to Evidence-Based Birth, “ultrasounds are right about half the time and wrong about half the time,” with weight estimates falling anywhere between 15% above and 15% below a baby’s actual weight (that’s between 6 lbs. 13 oz. and 9 lbs. 3 oz. for an actual 8 lb. baby!) (https://evidencebasedbirth.com/wp-content/uploads/2018/01/Big-Babies-Handout.pdf) And while 1 in 3 women is told she is having a big baby following a third-trimester ultrasound, only 1 in 10 will actually go on to have one. Suspicion alone of a large baby results in an increased likelihood that a provider will diagnose your labor as a “failure to progress” and encourage a c-section.
PELVIMETRY AND PELVIC DIMENSIONS
According to Anne Frye, the concern surrounding pelvic dimensions in pregnancy and birth peaked around the turn of the 19th century. During this time, many women worked from childhood into their childbearing years in darkened factories, had very little exposure to sunlight, and consumed diets deficient in Vitamin D, as well as many other essential nutrients. As a result, many women's pelvises during this time period were deformed by rickets, a condition resulting from lack of Vitamin D that leads to weak, soft bones in the body's weight-bearing structures (i.e the spine, pelvis, and legs). Women frequently had difficult, even deadly childbearing experiences as they attempted to vaginally birth babies through pelvises misshapen by rickets. Because surgical birth was extremely risky and carried with it a high mortality rate, providers had to devise a non-invasive and reliable way to assess a woman's pelvic dimensions. Enter pelvimetry, "the art of assessing the size and shape of a woman's pelvis and determining how well-suited her pelvis is to childbearing (Frye, Holistic Midwifery)."
Just like ultrasound and other diagnostic techniques, pelvimetry cannot definitively determine what size of baby a woman is capable of birthing. There is no magic dimension for an 8-pound baby versus a 10-pound baby. The purpose of pelvimetry, rather, is to ascertain the nuances of each individual pelvis and an idea of the size, shape, and flexibility of the structures. Specific information gained through pelvimetry includes:
Palpability of the sacral promontory
Concavity of the sacrum
Mobility of the coccyx
Shape of the pelvic cavity sidewalls
Prominence of and space between the ischial spines (bispinous diameter)
Angle of the pubic arch (pubic symphysis)
Space between the ischial tuberosities (bituberous diameter)
Because the pelvis is comprised of four bones joined by cartilage, it becomes malleable under the influence of pregnancy hormones, thus creating its ability to accommodate various fetal sizes and presentations. Performing pelvimetry in the third trimester not only provides valuable insight into the pelvic dimensions but can also indicate where progress may be prolonged and what positions may be most beneficial to assist in fetal descent. The second picture above illustrates the average dimensions of a gynaecoid pelvis. There are four categories of pelvises, with gynaecoid considered the most favorable for birth and android (the pelvic type found most commonly in males) being the least as it has the most narrow pelvic outlet and the most prominent ischial spines. Most female pelvises are some combination of the four types: gynaecoid, anthropoid, android, and platypelloid. As the table illustrates, babies typically take the path of least resistance during birth. There are less-common variable presentations such as transverse lie, variations of breech, brow and face presentations, etc. which sometimes make vaginal birth more difficult or impossible, but the majority of babies follow what are collectively known as the "cardinal movements of labor," two of which are specific to the pelvic dimensions:
Enter the pelvic inlet in a transverse (facing left or right) orientation as that is typically the largest dimension (13 cm.) and flex the neck.
Rotate in the pelvic cavity or bowl into an anterior-posterior (AP, or front- or back-facing) orientation. While the pelvic cavity is roughly 12 cm. all the way around, the baby moves from the cavity to the outlet, whose diameter is largest from front to back (13 cm.).
From here, the baby is positioned to pass under the pubic arch (pubic symphysis) and complete the additional cardinal movements leading to birth. It is important to note that the average female pelvis at its smallest dimension is approximately 11 cm. A flexed fetal head presenting in the occiput anterior (back of the head first) measures approximately 9.5 cm. Variations in fetal size and presentation do occur, but many women are able to deliver babies who are breech, occiput posterior (back of the head facing the mother's back), and other complex presentations such as face and nuchal hand with a provider who is competent and patient.
CONCLUSION
Knowing the individual nuances of a woman's pelvis is not only informative for a provider, but it can also be empowering for the mother. It can create a sense of confidence and trust in her body and her own ability to give birth, especially if she had a previous c-section due to "failure to progress." It can also be the difference between vaginal birth and a cesarean. For example, a woman who has a narrower pelvic outlet could require extra time for the baby's head to sufficiently mold to its shape during stage two. Information gleaned through pelvimetry can offer key pieces of knowledge. Encouraging its practice as part of routine prenatal care could reduce the instance of surgical and instrumental births performed due to what might be a blanket and inaccurate diagnosis.