The pelvic inlet is the upper boundary of the lesser, or true, pelvis, creating a separation from the greater, or false, pelvis above it. It functions as the primary entry point to the birth canal. Its shape and size are fundamental in determining the available space for a fetus to descend during childbirth. The dimensions of the inlet are a major focus in obstetrics because they represent the first bony constraint encountered by the descending fetal head.
Anatomical Structures Defining the Inlet
The pelvic inlet’s circumference is defined by a continuous bony ridge known as the linea terminalis. This line begins posteriorly at the sacral promontory, which is the prominent anterior projection of the first sacral vertebra. From there, the boundary extends laterally across the wings, or alae, of the sacrum.
The line then follows the arcuate line, a smooth ridge on the inner surface of the ilium bone. Moving forward, the border continues along the pectineal line of the pubic bone. The circumference is completed at the front by the pubic crest and the upper margin of the pubic symphysis.
The Standard Female Pelvic Inlet Shape
The typical, most common shape of the female pelvic inlet is classified as Gynacoid, which is generally described as being transversely oval or slightly rounded. This shape has evolved to be the most accommodating for human childbirth, contrasting with the often triangular or heart-shaped inlet of the male (Android) pelvis. The Gynacoid inlet is characterized by a transverse diameter that is slightly longer than its anteroposterior diameter.
The anteroposterior diameter, known as the true conjugate, runs from the sacral promontory to the upper inner edge of the pubic symphysis, typically measuring around 11.0 centimeters. However, the obstetric conjugate, which is the narrowest fixed anteroposterior measurement, is slightly shorter, averaging about 10.5 centimeters, as it is measured to the most protruding point on the back of the symphysis pubis.
The maximum transverse diameter is the widest point of the inlet, spanning approximately 13.0 centimeters between the farthest points on the arcuate lines. This greater transverse measurement means that the fetal head must typically engage and pass through the inlet in a sideways position to utilize the widest available space. The oblique diameters, running diagonally from the sacroiliac joint to the opposite iliopectineal eminence, generally measure about 12.0 centimeters. This allows for the necessary rotation of the fetal head as it navigates the birth canal.
Variations in Pelvic Inlet Shape
The Gynacoid shape represents the ideal, but the female pelvis is not uniform, showing considerable variation that is categorized by the Caldwell-Moloy classification system developed in the 1930s. This system identifies four parent types based on the geometric shape of the inlet, which reflect a spectrum of anatomical possibilities. While the Gynacoid is the most frequent type, found in nearly half of all women, three other distinct shapes exist.
The Android pelvis, often considered the male pattern, presents an inlet that is distinctly heart-shaped or triangular, with the widest transverse diameter situated closer to the sacrum. This configuration tends to shorten the anteroposterior space and create a narrow anterior apex, which is less favorable for descent during labor.
The Anthropoid pelvis is characterized by an inlet that is a long, narrow oval, resembling an egg on its end. In this type, the anteroposterior diameter is actually longer than the transverse diameter, a reversal of the Gynacoid shape. This long-oval configuration is sometimes described as being ape-like and requires the fetal head to engage in a direct anteroposterior orientation.
The least common type is the Platypelloid pelvis, which features an inlet that is flat and broad, often described as a flat oval or kidney shape. Here, the transverse diameter is significantly longer than the anteroposterior diameter, meaning the inlet is wide but severely shallow. Most individuals possess a pelvis with mixed characteristics, falling somewhere between these four primary parent types.