The second stage of labor, often called the pushing stage, is the final phase of childbirth where the cervix is fully dilated and the baby moves through the birth canal. This powerful process requires a synchronized effort between two distinct types of muscle groups: those that act automatically and those that are consciously controlled by the birthing person. Understanding this anatomical partnership, which involves both involuntary force and voluntary physical exertion, can help demystify the mechanics of delivery.
The Uterus: The Involuntary Driver
The primary force driving the baby downward is generated by the uterus, a muscular organ composed mainly of smooth muscle tissue called the myometrium. These muscles contract rhythmically and powerfully without any voluntary input.
Uterine contractions serve a dual purpose during the second stage of labor. They provide constant, expulsive pressure that pushes the baby against the open cervix and into the pelvis. With each contraction, the uterine muscles also undergo retraction, where they shorten slightly without fully returning to their original length, effectively pulling the uterus up over the descending baby.
This automatic action is a physiological reflex designed to ensure the baby’s progression. The uterine muscle provides the sustained power that the voluntary muscles augment.
Voluntary Muscles for Generating Downward Force
The muscles a person uses to push are accessory muscles of labor, and their coordinated contraction significantly increases the expulsive power. This voluntary effort is achieved by generating high intra-abdominal pressure, similar to bearing down during a bowel movement. The diaphragm, the large dome-shaped muscle beneath the lungs, plays a central role. When a person takes a deep breath and pushes, fixing the diaphragm in a lower position, the muscle acts like a piston, pushing down on the abdominal contents.
This downward force is amplified by the contraction of the abdominal wall muscles, which compress the uterus and its contents. The Rectus Abdominis, the long muscles running vertically down the front of the abdomen, contract to flex the trunk and squeeze the abdominal cavity. The Transversus Abdominis, the deepest layer of abdominal muscle, acts like a corset to stabilize the core and further compress the abdomen. The Oblique muscles, which wrap around the sides, also contribute to the overall squeeze. The combined effort of the fixed diaphragm and the contracting abdominal wall muscles can increase intrauterine pressure by up to 62%, substantially assisting the delivery process.
The Role of the Pelvic Floor and Deep Core
Contrary to common belief, the muscles of the pelvic floor are not actively used to push the baby out; their function is to yield, stretch, and guide. The pelvic floor is a hammock-like group of muscles, including the Levator Ani group, that forms the base of the pelvis. As the baby’s head descends, these muscles must relax and lengthen to allow passage, often stretching to two or three times their resting length. If the pelvic floor muscles are tense or contracted, they create resistance that can prolong the second stage of labor and increase the risk of tissue tearing.
Simultaneously, the deep core muscles, such as the multifidus along the spine, work to stabilize the torso against the immense pressure created by the diaphragm and abdominal muscles. This spinal stabilization ensures that the pushing force is efficiently directed downward toward the pelvic outlet.
Coordinating the Push: Techniques and Positioning
Effective pushing requires coordinating the voluntary forces (diaphragm and abdominal muscles) with the involuntary contractions (uterus) while ensuring the relaxation of the pelvic floor. There are two main approaches to pushing: directed and physiological.
Directed pushing, or coached pushing, often involves holding the breath and bearing down forcefully for a sustained period, typically 10 seconds, three or four times per contraction. This technique maximizes intra-abdominal pressure but can sometimes lead to straining and may be counterproductive if the pelvic floor is not relaxed. Physiological or spontaneous pushing encourages the birthing person to follow their body’s natural urge to bear down, which may involve shorter, more frequent pushes or a more breath-regulated effort.
This often employs an open-glottis technique, where the person exhales with sound while pushing, which helps to prevent excessive pressure and encourages pelvic floor flexibility. Changing positions also significantly impacts the coordination of these muscles and forces.
Upright positions like squatting or hands-and-knees utilize gravity to assist the downward force, while side-lying can help open the hips and reduce the risk of tearing.