The pelvis is made up of the two innominate bones (which occupy the front and sides) and the sacrum and coccyx (which are behind). The bones articulate through four joints. The sacroiliac joint is the most important, linking the sacrum to the iliac part of the innominate bones. The symphysis of the pubis joins the two pubic bones. The sacrococcygeal joint attaches the sacrum to the coccyx.
The false pelvis lies above the true pelvis, superior to the linea terminalis. Its only obstetric function is to support the enlarged uterus during pregnancy. Its boundaries are:
Posteriorly: lumbar vertebrae
Laterally: iliac fossae
Anteriorly: anterior abdominal wall
The true pelvis (Fig. 5-1A) lies below the pelvic brim, or linea terminalis, and is the bony canal through which the fetus must pass. It is divided into three parts: (1) the inlet, (2) the pelvic cavity, and (3) the pelvic outlet.
The inlet (pelvic brim) is bounded:
Anteriorly by the pubic crest and spine
Laterally by the iliopectineal lines on the innominate bones
Posteriorly by the anterior borders of the ala and promontory of the sacrum
The pelvic cavity (Fig. 5-1B) is a curved canal.
The anterior wall is straight and shallow. The pubis is approximately 5 cm long
The posterior wall is deep and concave. The sacrum is approximately 10 to 15 cm long
The ischium and part of the body of the ilium are found laterally
The pelvic outlet is diamond shaped. It is bounded:
Anteriorly by the arcuate pubic ligament and the pubic arch
Laterally by the ischial tuberosity and the sacrotuberous ligament
Posteriorly by the tip of the sacrum
The pelvic inclination (Fig. 5-1C) is assessed when the woman is in the upright position. The plane of the pelvic brim makes an angle of about 60° with the horizontal. The anterior superior iliac spine is in the same vertical plane as the pubic spine.
The axis of the birth canal (Fig. 5-1D) is the course taken by the presenting part as it passes through the pelvis. At first it moves downward and backward to the level of the ischial spines, which is the area of the bony attachment of the pelvic floor muscles. Here the direction changes and the presenting part proceeds downward and forward.
The pelvic planes (Fig. 5-2) are imaginary flat surfaces passing across the pelvis at different levels. They are used for the purposes of description. The important ones are as follows:
Log In to View More
If your institution is currently a subscriber
of the ObGyn Collection please sign in below.
If your institution is not a subscriber
please click here
to learn more.
Want remote access to your institution's subscription?
Sign in to your MyAccess profile while you are actively authenticated on this site via your institution (you will be able to verify this by looking at the top right corner of the screen - if you see your institution's name, you are authenticated). Once logged in to your MyAccess profile, you will be able to access your institution's subscription for 90 days from any location. You must be logged in while authenticated at least once every 90 days to maintain this remote access.
If your institution subscribes to this resource, and you don't have a MyAccess profile, please contact your library's reference desk for information on how to gain access to this resource from off-campus.
Pop-up div Successfully Displayed
This div only appears when the trigger link is hovered over.
Otherwise it is hidden from view.