Normal Value of External Sacrosciatic Diameter
May 22, 2022
Source: Cainiu Health
Dr. Lin Yunfei
Introduction
The normal external conjugate (sacroshameal diameter) measures 18–20 cm. This measurement is a key parameter in external pelvic assessment of pregnant women. The woman lies on her left side, with the right leg extended and the left leg flexed. The clinician then measures the distance from the spinous process of the fifth lumbar vertebra to the midpoint of the superior border of the pubic symphysis. The external conjugate provides an estimate of the anteroposterior diameter of the pelvic inlet and is highly valuable in determining the mode of delivery.
External conjugate (EC): The pregnant woman lies on her left side, with the right leg extended and the left leg flexed. The distance is measured from the spinous process of the fifth lumbar vertebra (L5) to the midpoint of the superior border of the pubic symphysis. What is the normal value for the external conjugate?
Normal Value of the External Conjugate
The normal value for the external conjugate is 18–20 cm. This measurement is one of the most important parameters in external pelvic assessment during pregnancy. To perform the measurement, the pregnant woman lies on her left side with the right leg extended and the left leg flexed, while the clinician measures the distance from the spinous process of the fifth lumbar vertebra (L5) to the midpoint of the superior border of the pubic symphysis.
The external conjugate provides an indirect estimate of the anteroposterior diameter of the pelvic inlet and is highly valuable in determining the mode of delivery. An external conjugate of 16.5–17.5 cm generally permits a trial of vaginal delivery, provided the mother is in good general health, has adequate uterine contractility, and the term live fetus weighs no more than 3,000 g (≈6 Chinese jin); cesarean delivery may be required if labor progress is unsatisfactory. An external conjugate <16 cm suggests narrowing of the pelvic inlet, making it highly unlikely that a term live fetus will engage and descend adequately for vaginal delivery—cesarean delivery is therefore indicated.
The spinous process of the fifth lumbar vertebra corresponds anatomically to the upper angle of the rhomboid fossa (also known as Michaelis’ rhomboid). This measurement indirectly estimates the anteroposterior diameter of the pelvic inlet and represents the most critical parameter in external pelvic assessment—helping determine whether a trial of vaginal delivery is feasible. Avoid staying up late at night; maintain regular sleep-wake cycles and limit prolonged use of mobile phones or computers. We hope this information is helpful to you!
