A 26-year-old primigravida at 39 weeks' gestation is admitted to the labor unit and progresses to the active phase of the second stage of labor. Her pregnancy has been uncomplicated. To successfully navigate the maternal bony pelvis, the fetus must undergo a series of positional changes known as the cardinal movements of labor. Assuming a fetus in a normal occiput anterior presentation, arrange the following cardinal movements in the correct physiological and chronological sequence, beginning with the fetal head entering the pelvis.
- 1Engagement: The biparietal diameter of the fetal head passes through the maternal pelvic inlet.
- 2Flexion: The fetal cervical spine flexes, bringing the chin toward the thorax.
- 3Internal rotation: The fetal occiput rotates toward the maternal pubic symphysis.
- 4Extension: The fetal head deflects as it pivots under the pubic symphysis.
- 5External rotation (restitution): The delivered head rotates anatomically.
Answer
The normal physiological sequence of the cardinal movements of labor for an occiput anterior presentation is Engagement, followed by Flexion, Internal rotation, Extension, and finally External rotation (restitution).
The cardinal movements of labor describe the physiological and biomechanical changes in fetal position required to successfully traverse the maternal pelvis. For a fetus in a vertex, occiput anterior position, the sequence begins with Engagement of the head into the inlet. Resistance from the pelvic structures causes Flexion, minimizing the presenting diameter. As the head descends, Internal rotation aligns the sagittal suture with the anteroposterior axis of the pelvic outlet. The curve of the birth canal causes Extension, leading to the delivery of the head. Finally, External rotation (restitution) untwists the fetal neck, aligning the delivered head with the shoulders in preparation for expulsion.
Step-by-Step Solution
Key Concept
The cardinal movements of normal labor (mechanism of labor).