How to tell the size of your pelvis

The fetus must pass through the pelvis when it is delivered from the mother, the so-called "bone birth gate". Whether the mother's delivery is successful is closely related to the size and shape of the pelvis. The size and shape of the pelvis vary from person to person. Different health conditions, physical conditions, genetic factors, and race lead to differences in the size and shape of the pelvis between individuals. So how do you determine the size of a pregnant woman’s pelvis?

How to tell the size of a pregnant woman's pelvis

Pregnant mothers cannot accurately measure their pelvis by themselves and need to go to the hospital for pregnancy check-up to measure it accurately. The methods and measuring instruments for accurate external and internal pelvic measurements are different. At present, manual measurement is often used.

1. Measurement method of vertex diameter: Vertex diameter is also known as sacral nominal diameter, which refers to the distance from the sacral promontory to the lower part of the ischial tuberosity. Normally it should be more than 11.5 cm. When measuring, let the pregnant woman lie flat on the bed, bend her legs and hold her knees with both hands, so that the roots of her thighs are close to the abdominal cavity and the buttocks are raised for easy examination. The operator wears gloves, straightens the middle and index fingers and inserts them into the vagina with a light rhythm, searches for the sacral promontory along the front of the sacrum from top to bottom, and presses the base of the fingers slightly towards the perineum.

The sacral promontory of a normal pelvis should not be palpable, indicating that the vertex diameter is greater than 11.5 cm. If the sacral promontory can be touched, the tip of the operator's middle thumb should firmly touch the sacral promontory, and the ring finger should be placed directly below the ischial tuberosity, and the ring finger of the other hand should be used to mark the point, and then the finger should be withdrawn. The distance from the mark to the tip of the middle thumb is the length of the vertex diameter. If the operator's fingers are shorter (the length from the tip of the middle thumb to the palm is less than 11.5 cm), a finger guard made of a bottle cork can be put on the end of the middle thumb.

2. Accurate measurement inside the pelvis: If there are abnormalities in the measurements on both sides of the pelvis, accurate measurement inside the pelvis should be carried out. The vertex diameter is <11.5cm, and the sacral promontory is prominent, indicating that the pelvic passage is narrow and the pelvis is relatively flat. A narrow mid-pelvic plane and a narrow pelvic entrance plane often exist at the same time. The anterior curvature of the sacrum, the diameter between the sciatic spines, and the width of the sciatic notch (i.e. the width of the sacrospinous cruciate ligament) should be accurately measured.

If the intersciatic spine diameter is <10 cm and the sciatic notch width is <2 horizontal fingers, the mid-pelvic plane is narrow. If the ischial tuberosity diameter is less than 8 cm, the posterior sagittal diameter of the inlet should be measured and the range of motion of the sacrococcygeal joint should be checked to estimate the narrowness of the pelvic inlet plane. If the sum of the ischial tuberosity diameter and the posterior sagittal diameter of the inlet and outlet is less than 15cm, the pelvic inlet and outlet plane is narrow.

Best time for accurate pelvic measurement

In clinical medicine, each hospital outpatient clinic is slightly different. Some hospitals measure the pelvis during the consultation, most hospitals measure the pelvis between 28 and 34 weeks of pregnancy, and some hospitals also do an assessment at 37 to 38 weeks of pregnancy (including internal measurement of the pelvis after private parts disinfection or measurement of the pelvis through the anus). X-ray pelvic measurement is not recommended to determine whether the fetus can be delivered naturally, nor is late-stage orthopaedic measurement.