In late pregnancy, extra-pelvic and intra-pelvic measurements are required to determine if a woman is able to deliver vaginally.
1. Extra-pelvic measurements
(1) Inter-iliac spine diameter and inter-iliac crest diameter: Pregnant women take the supine position and measure the distance between the outer edge of the anterior superior iliac spine and the outer edge of the widest point of the iliac crest on both sides.
(2) Sacro-pubic external diameter: the distance from the midpoint of the upper edge of the pubic symphysis to the lower edge of the spinous process of the 5th lumbar vertebra was measured with the right leg straight and the left leg flexed.
(3) Intersciatic tubercle diameter: the distance between the medial margins of the two sciatic tubercles was measured with the pregnant woman’s legs flexed and her hands clasped around her knees, spreading them outward and upward on both sides.
(4) Angle of the pubic arch: the doctor will put the tips of the thumbs of the right and left hands diagonally together and place them on the lower edge of the pubic symphysis of the pregnant woman, and then place the thumbs flat on the descending branch of the pubic bone, and measure the angle formed.
2. Intra-pelvic measurement
(1) Diagonal diameter: the doctor inserts the finger and middle finger into the vagina of the pregnant woman, the tip of the finger touches the midpoint of the upper edge of the sacral promontory, and the upper edge of the finger is close to the lower edge of the pubic symphysis, and the finger of the other hand indicates the point of contact, and then measures the distance from the tip of the middle finger to the point marked on the finger.
(2) Inter-sciatic spine diameter: The doctor inserts the index and middle fingers into the vagina of the pregnant woman and touches the sciatic spines on both sides of the woman to estimate the distance between them.
(3) Posterior Sagittal Diameter: The doctor extends the index finger of the right hand toward the anus of the pregnant woman toward the sacrum, the thumb is placed outside the body at the sacrococcygeal area, and the two fingers work together to locate the tip of the sacrum, and one end of the pelvic outlet measuring device is placed at the midpoint of the intersciatic tuberosity diameter, and the other end is placed at the tip of the sacrum.
If the pelvic measurement in late pregnancy reveals values significantly out of the normal range, the most appropriate mode of delivery should be decided under the guidance of a doctor.