North American Registry of Midwives (NARM) Practice Exam

What is the smallest pelvic diameter that the fetus must accommodate itself?

1. Subpubic arch

2. Interspinous diameter

3. Biischial diameter

4. Pelvic inlet

The correct answer is: Interspinous diameter

The interspinous diameter is the smallest pelvic diameter that the fetus must accommodate itself during the birthing process. This diameter is measured between the ischial spines within the pelvic cavity and is crucial for guiding the passage of the fetus through the birth canal. It serves as a significant constriction point in the pelvic anatomy, and the fetus must navigate this space as it descends during labor. Understanding the dimensions of the interspinous diameter highlights its role in determining whether a vaginal birth can occur. If the diameter is too small relative to the size of the fetus, complications such as obstructed labor may arise. This anatomical feature is critical not just for the successful delivery of the fetus, but also for assessing potential interventions that may be necessary in cases of a disproportionate head-to-pelvis size. Other diameters, while important for overall pelvic assessment, do not represent the constricted space the fetus must navigate as it progresses toward delivery. Therefore, the interspinous diameter is singled out as the key measurement for effective accommodation of the fetus during labor.