How do you know the size of your pelvis?

The size of the pelvis is very important for women, especially for those in the late stages of pregnancy, because only when the pelvis is of the right size can a natural birth be possible. If the pelvis is too small, then the only option is a caesarean section. The size of your pelvis can be measured in many ways, and there are also optimal times to measure it. Here we will introduce the measurement method and the best time to measure the size of the pelvis!

Interiliac spine diameter: The pregnant woman lies on her back and uses a pelvic measuring tape to measure the distance between the outer edges of the two anterior superior iliac spines. The normal value is about 23 to 25 cm.

Iliac crest diameter: The pregnant woman lies on her back and measures the widest distance between the outer edges of the two iliac crests. The normal value is about 25 to 28 cm.

Sacropubic external diameter: The mother lies on her side with her upper leg straight and her lower leg bent. The distance from the midpoint of the upper edge of the pubic symphysis to the lower part of the spinous process of the fifth lumbar vertebra is measured. The normal value is 18 to 20 cm. This diameter can indirectly infer the size of the anterior-posterior diameter of the pelvic inlet.

Intersciatic tuberosity diameter: the distance between the inner sides of the two ischial tuberosities, with a normal value of 8.5 to 9.5 cm, representing the transverse diameter of the pelvic outlet.

The biparietal diameter is most closely related to your transverse birth canal diameter. The biparietal diameter is the largest part of your child, while the transverse birth canal diameter is the smallest part of the birth canal.

1. Measurement method of diagonal diameter: The diagonal diameter is also called the sacropubic internal diameter, which refers to the distance from the sacral promontory to the lower edge of the pubic symphysis, and should normally be greater than 11.5 cm. When measuring, let the pregnant woman lie on her back on a flat bed, bend her legs and hold her knees with both hands, so that her thighs are close to the abdominal wall and her buttocks are raised for easy examination. The examiner wears gloves and gently inserts the middle and index fingers into the vagina, searches for the sacral promontory along the front of the sacrum from bottom to top, and presses the base of the fingers slightly toward the perineum.

The sacral promontory of a normal pelvis cannot be palpated, indicating that the diagonal diameter is greater than 11.5 cm. If the sacral promontory can be touched, the tip of the examiner's middle finger should be in close contact with the sacral promontory, and the index finger should be placed just below the pubic symphysis. The index finger of the other hand should be used to make a mark at that location, and then the finger should be withdrawn. The distance from the mark to the tip of the middle finger is the length of the diagonal diameter. If the examinee's fingers are short (the length from the tip of the middle finger to the base of the thumb is less than 11.5 cm), a finger cuff made of cork can be put on the end of the middle finger.

2. Internal pelvic measurement: If abnormalities are found in the lateral pelvic measurement, internal pelvic measurement should be performed. The diagonal diameter is <11.5cm, the sacral promontory is prominent, the pelvic inlet plane is narrow, and it is a flat pelvis. Midpelvic plane stenosis and pelvic outlet plane stenosis often exist at the same time. The anterior curvature of the sacrum, the intersciatic diameter, and the width of the sciatic notch (i.e., the width of the sacrospinal ligament) should be measured.

If the intersciatic spine diameter is <10cm and the sciatic notch width is <2 horizontal fingers, it is considered a narrow mid-pelvic plane. If the inter-ischial tuberosity diameter is <8 cm, the posterior sagittal diameter of the outlet should be measured and the sacrococcygeal joint mobility should be checked to estimate the degree of narrowing of the pelvic outlet plane. If the sum of the inter-ischial tuberosity diameter and the posterior sagittal diameter of the outlet is less than 15 cm, the pelvic outlet plane is narrow.

Clinically, each hospital is different. Some hospitals measure the pelvis during the initial diagnosis, most hospitals measure the pelvis between 28 and 34 weeks of pregnancy, and some hospitals perform an assessment at 37 to 38 weeks of pregnancy (including internal pelvic measurement after vulva disinfection or transanal pelvic measurement). X-ray pelvic measurement is not recommended to determine whether the fetus can be delivered vaginally, nor is late orthopedic internal measurement.