The true pelvis is divided into three regions known as the pelvic brim, the cavity and the outlet. The measurements of each of these regions is important as the fetal head has to negotiate its way through the pelvis to be born. The midwife should be familiar with the dimensions in order to be able to recognise any abnormality in the size or shape of the pelvis that mey affect the care given to the woman.
Diameters of the brim.
Anteroposterior diameter. This diameter extends from the sacral promotory to the upper inner border of the symphysis pubis and measures approximately 11cms. It is also called the obstetrical conjugate. It is the most important of the pelvic measurements as it is the first boney strait through which the fetus has to pass. The anatomical of true conjugate is measured from the promotory of the sacrum to the centre of the upper surface of the symphysis pubis and measures approximately 12cm. Although it is slightly longer than the obstetric conjugate, the extra space is not available for the passage of the fetus.
The right obliquie diamter passes from the right sacroiliac joint to the left iliopectineal eminence and the left oblique extends from the left sacroiliac joint to the right iliopectineal eminence. Each measures about 12cms. The transverse diameter is between the widest points on the iliopectineal lines and measures approximately 13cms. The sacrocotyloid diameter is measured from the sacral promotory to the iliopectineal eminence, on the same side, and is approximately 9cms. The diagonal conjugate is measured antero-posteriorly from the apex of the pubic arch of the symphysis to the sacral promotory. It measures about 1.25cm more than the obstetric conjugate and may be felt during a vaginal examination as part of a pelvic assessment. It is unlikely that it would be reached by examining fingers unless the pelvis is small and where the measurement is less than 12cm.
Diameters of the cavity The cavity extends from the brim above to the outlet below. It is almost circular in shape. The anterior wall is formed by the pubic bones and symphysis pubis and its depth is 4cm. The posterior wall is formed by the curve of the sacrum which is 12cm in length. The diameters, similar in direction to the brim, are all considered to be 12cm when measured.
Diameters of the outlet The anteroposterior diameter extends from the sacrococcegeal joint to the lower border of the symphysis pubis and measures 13cm. The oblique diameters pass obliquely forwards from the sacrospinous ligaments and cannot be accurately measured. They are considered to be about 12cm. The transverse diamter of the outlet measures 11cm.
The pelvic outlet is slightly smaller than the pelvic brim, but it would be unusaual for a fetal head to be able to pass through the brim and not be able to pass through the outlet. The fetal head will usually enters the pelvis with its long diamteter transversely across the brim, rotates in the cavity and emerges from the outlet with its long diameter anteroposterior.To test your knowledge of the diameters of the pelvis visit student activity 2
Axis of the Birth Canal This describes the direction in which the fetus progresses as it passes through the birth canal.
An axis is a line that passes through the centre of a plane; A plane is an imaginary flat surface.
Pelvic planes: When a woman stands upright the pelvis slopes quite steeply referred to as pelvic inclination. The angle is between the plane of the brim and the anterior surface of fifth lumbar vertebra. It usually measures about 135°. The pelvic planes are imaginary flat surfaces at the brim, cavity and outlet of the pelvic canal.
Plane of pelvic brim: The anterior superior iliac spines are immediately above the symphysis pubis and this plane slopes at an angle of 55°. The angle between the plane of the brim and the anterior surface of the first sacral vetebra is the sacral angle. It usually measures about 90°.
Plane of the cavity: This plane lies across the midpoint of the sacrum and pubic bone.Plane of the outlet: This slopes at an angle of 15° and is almost horizontal.
The subpubic angle is that which is between the two inferior pubic rami and forms the pubic arch. In a normal gynaecoid pelvis this should be 90°.
In a normal labour the fetal head will follow the axis of each plane. It will descend in a straight line, downward and backward, through the plane of the brim and cavity till it reaches the ischial spines. The fetal head is then deflected by the pelvic floor, at the curve of Carus, to continue its progress downward and forward culminating in delivery of the head.
To watch how the fetus negotiates the pelvis and the interplay of the various angles, planes and measurements go to activity 4.