How to measure the pelvis

Whether the pelvis is normal has an extremely important relationship with whether the pregnant woman can give birth naturally. Therefore, when determining whether the pregnant woman should choose natural birth or caesarean section, it is necessary to first measure the pregnant woman's pelvis. This will enable you to better understand the condition of the pregnant woman's pelvis and birth canal. If the size of the pelvis is normal, you can choose to give birth naturally. But if the pelvis is too narrow, caesarean section is the only option.

External pelvic measurements are made using a pelvic outlet gauge. When measuring, the examiner faces the pregnant woman's vulva and mainly measures and records the following items.

(1) Interiliac spine diameter (IS): The pregnant woman lies supine on the examination bed with her legs stretched out, and the distance between the lateral edges of the anterior superior iliac spines on both sides is measured and the data is recorded. The normal value is 23 to 26 cm.

(2) Interiliac crest diameter (IC): The pregnant woman lies on her back with her legs stretched out on the examination bed, and the widest distance between the outer edges of the iliac crests on both sides is measured and the data is recorded. Measuring the above two items can indirectly understand the transverse diameter length of the pelvic inlet. The normal value is 23 to 26 cm.

(3) Sacropubic external diameter (EC): The pregnant woman lies on her left side with her right leg straight and her left leg bent. The EC is measured as the distance from the midpoint of the upper edge of the pubic symphysis to the depression below the spinous process of the fifth lumbar vertebra. This diameter can indirectly infer the anteroposterior diameter of the pelvic inlet and is the most important diameter in external pelvic measurement. The sacropubic external diameter is related to the thickness of the bone. The measured sacropubic external diameter minus 1/2 of the radial ulna circumference is equivalent to the anterior-posterior diameter of the pelvic inlet. The normal value is 18 to 20 cm.

(4) Transverse diameter of the outlet (inter-ischial tuberosity diameter): Lie on your back with your legs bent and your hands hugging your knees, fully flexing the hip and knee joints; measure the distance between the inner edges of the two ischial tuberosities. The normal value is 8.5 to 9.5 cm.

(6) Pubic arch angle: Lie on your back with your legs bent and your hands hugging your knees. Place the tips of your left and right thumbs together diagonally at the lower edge of the pubic symphysis, with your left and right thumbs flat on the descending ramus of the pubic bone. Measure the angle between the two thumbs and record it. The normal value is 90°.

2. Measurement inside the pelvis

The measurement inside the pelvis uses a mid-pelvis measuring device. The pregnant woman lies on her back on a flat bed with her legs bent and her knees held with both hands, so that her thighs are close to the abdominal wall and her hips are raised for easy inspection. The examiner wears gloves. The following items are mainly measured in pelvic measurements.

(1) Diagonal diameter (DC): The examiner inserts the tip of the middle finger into the vagina and touches the midpoint of the upper edge of the sacral promontory. The upper edge of the index finger is close to the lower edge of the pubic symphysis. The index finger of the other hand is used to correctly mark this contact point. The finger is withdrawn from the vagina and the distance from the tip of the middle finger to this contact point is measured. This is the diagonal diameter. It is the distance from the lower edge of the pubic symphysis to the midpoint of the upper edge of the sacral promontory. The normal value is 12.5 to 13 cm. This value minus 1.5 to 2 cm is the anteroposterior diameter of the pelvic inlet, also known as the true fusion diameter.

(2) Diameter between ischial spines (BD): The measurement method is to place the index and middle fingers of one hand into the vagina, touch the ischial spines on both sides respectively, and estimate the distance between them. That is, measure the distance between the two ischial spines. The normal value is about 10cm.

(3) Sciatic notch width: represents the posterior sagittal diameter of the mid-pelvis. Its width is the distance between the ischial spine and the lower part of the sacrum, that is, the width of the sacrospinal ligament. Place the index finger in the vagina and move it over the ligament. It is normal if it can accommodate 3 horizontal fingers (5.5-5.6cm), otherwise it is considered as middle finger pelvic stenosis.

In addition to manual measurement with the help of a measuring instrument, pelvic measurement can also be performed using ultrasound to measure the sacropubic diameter during the second trimester to understand the pelvic condition of the pregnant woman.

The pregnant woman lies supine on the examination bed. The sacral promontory is displayed in the midline sagittal section of the lower abdomen, and the pubic symphysis is displayed in a transverse section above the pubic symphysis. The probe is rotated 90° at the midpoint of the pubic symphysis to make the probe consistent with the longitudinal axis of the pregnant woman. A curved arc-shaped bone interface can be seen behind the uterus. The more convex part is the sacral promontory. Move the probe up and down so that the scanning direction is slightly upward. Freeze the image when the echo of the sacral promontory and the pubic symphysis appear in the same plane. The distance between the inner side of the midpoint of the upper edge of the pubic symphysis and the narrowest part of the line connecting the front end of the sacral promontory is the sacropubic diameter, which is also the anterior-posterior diameter of the entrance.