The human pelvis is a bony ring structure that supports the upper body and protects the organs within the pelvic cavity. Its size and shape vary significantly among individuals, especially between biological sexes. The female pelvis is generally wider and shallower than the male pelvis, an adaptation to accommodate pregnancy and childbirth. Understanding the specific shape of the pelvis is a key factor in obstetrics, as its dimensions directly influence the mechanism and potential ease of a vaginal delivery. Healthcare professionals use a standardized system to classify these variations and determine the safest path for birth.
The Four Primary Pelvic Classifications
In the 1930s, obstetricians William Caldwell and Howard Moloy established a classification system based on the shape of the pelvic inlet, dividing the female pelvis into four distinct types. The Gynecoid pelvis is the typical female shape, found in approximately half of all women, and is optimally suited for childbirth. Its inlet is slightly rounded or oval, the sidewalls are straight, and the subpubic arch is wide, typically measuring between 90 and 100 degrees. This geometry provides maximum space for the fetal head to descend and rotate easily through the birth canal.
The Android pelvis is present in about 20% of women and is characterized by a heart-shaped or triangular inlet. The sidewalls converge, creating a funnel shape, and the ischial spines tend to be prominent, narrowing the mid-pelvis. The subpubic arch is narrow, typically less than 90 degrees, which significantly reduces the available space at the outlet. This configuration can obstruct the fetus’s passage, leading to potential complications.
The Anthropoid pelvis, occurring in roughly 25% of women, is distinguished by an oval inlet that is long from front-to-back (anteroposteriorly) but narrow from side-to-side (transversely). This shape is deeper than the other types. While the transverse diameters are reduced, the elongated anteroposterior diameter can still allow for a vaginal delivery.
Finally, the Platypelloid pelvis is the least common type, found in only about 5% of women, and is characterized by a flat, wide shape. The inlet is a shallow, transverse oval, meaning it is much wider transversely than anteroposteriorly. The anteroposterior diameter is significantly shortened, which can present a unique obstacle for the fetal head’s engagement into the pelvis. These four classifications provide the anatomical baseline for predicting labor progression, though many individuals possess a pelvis that is a mix of two types.
Clinical Importance in Pregnancy and Childbirth
Pelvic dimensions and shape are medically significant because of their impact on the successful passage of the baby during delivery. The central concern is Cephalopelvic Disproportion (CPD), a condition where the baby’s head is too large, or the mother’s pelvis is too small or unfavorably shaped, to allow for a vaginal birth. While the Gynecoid pelvis is associated with the lowest risk of CPD, the other shapes present distinct challenges to the mechanism of labor.
In the Android pelvis, the heart-shaped inlet and narrow mid-pelvis often lead to a slow, difficult descent of the fetal head. The prominent ischial spines can prevent the necessary rotation of the baby, frequently resulting in a fetal malposition (occiput posterior). This lack of space and failure to rotate are common reasons for a prolonged second stage of labor and an increased likelihood of requiring an operative delivery or Cesarean section.
The Anthropoid pelvis, with its long anteroposterior diameter, encourages the fetal head to engage in an anterior or posterior position rather than the typical transverse position. Labor is often uncomplicated but may be longer, as the baby may not perform the standard internal rotation.
The Platypelloid pelvis, with its severely shortened front-to-back diameter, presents the greatest initial hurdle, as the fetal head may not engage with the pelvic inlet. This difficulty with engagement and descent can lead to an obstructed labor, making this shape the one most often associated with the need for a Cesarean delivery.
Methods for Professional Pelvic Assessment
Pelvimetry, the determination of pelvic shape and size, relies on both physical examination and modern imaging technology. The initial method is Clinical Pelvimetry, a manual assessment performed by a doctor or midwife during a vaginal examination, typically late in pregnancy. The examiner estimates the dimensions of the internal pelvis, including the prominence of the ischial spines and the angle of the pubic arch.
A key measurement taken during this manual assessment is the diagonal conjugate, which is the distance from the lower border of the pubic symphysis to the sacral promontory. If the sacral promontory cannot be reached with the fingers, the pelvis is considered adequate, with a diagonal conjugate estimated to be approximately 12.5 centimeters or more. This measurement is then used to estimate the true conjugate (or obstetric conjugate), the shortest diameter of the inlet, by subtracting 1.5 to 2 centimeters. Because this is a subjective estimation, clinical pelvimetry has limitations in providing precise measurements of the bony structure.
For more accurate measurements, especially when CPD is suspected or a trial of labor is planned after a previous Cesarean section, imaging techniques are employed. X-ray pelvimetry was historically used, but it is now largely obsolete in routine obstetrics due to the exposure of the fetus to ionizing radiation. The radiation risk did not justify its low predictive value.
Modern practice favors cross-sectional imaging. Computed Tomography (CT) pelvimetry provides rapid, precise measurements but still involves a minimal dose of radiation. The preferred method, particularly for the pregnant patient, is Magnetic Resonance Imaging (MRI) pelvimetry, as it is entirely radiation-free. MRI offers highly accurate measurements of all pelvic diameters and clearly delineates the bony anatomy, providing a reliable means to assess the pelvic architecture and plan the safest delivery strategy.