Forces of Labor
The forces that expel the fetus and its appendages from the uterus are collectively referred to as the forces of labor. These include uterine contractions (typically referred to as uterine force), abdominal wall and diaphragm contractions (commonly known as abdominal pressure), and levator ani muscle contractions.
Uterine Contraction Force
Uterine contraction force constitutes the primary driving force of labor and persists throughout the entire process of childbirth. During labor, uterine contractions facilitate the effacement of the cervical canal, dilation of the cervix, descent of the presenting fetal part, as well as the expulsion of the fetus and placenta. Normal uterine contractions during labor exhibit the following characteristics:
Rhythmicity
Rhythmic uterine contractions serve as a key marker of the onset of labor. Each contraction progresses from weak to strong (increment phase), sustains for a period (peak phase, typically lasting 30–40 seconds), and then wanes gradually (decrement phase) before ceasing, followed by an interval of rest. Rest intervals generally last 5–6 minutes. As labor progresses, the duration of contractions lengthens while the intervals shorten. Once full cervical dilation is achieved, contractions may last up to 60 seconds, with intervals reduced to only 1–2 minutes. This rhythm continues until delivery is complete.
Figure 1 Diagram illustrating the rhythmic pattern of normal uterine contractions during labor
At the peak of a contraction, intrauterine pressure increases to 40–60 mmHg near the end of the first stage of labor and rises further to 100–150 mmHg during the second stage. During intervals, pressure falls to 6–12 mmHg. The compression of uterine blood vessels during contractions reduces blood flow momentarily, but blood flow is restored during the intervals, which ensures adequate perfusion to the fetus.
Symmetry and Polarity
Normal uterine contractions originate from both uterine cornua, quickly concentrate at the fundal midline in a symmetrical manner, and then spread toward the lower uterine segment at a speed of approximately 2 cm/s. This symmetrical and coordinated pattern covers the entire uterus within about 15 seconds. Uterine contractions are strongest and most sustained at the fundus and gradually diminish toward the lower uterine segment, reflecting the polarity of contractions. The contraction intensity at the uterine fundus is approximately twice that of the lower uterine segment.
Figure 2 Symmetry and polarity of uterine contractile force
Retraction Mechanism
During each contraction, muscle fibers in the uterine body shorten and thicken. Even when relaxed during an interval, these fibers do not fully return to their original length. This progressive shortening of muscle fibers over repeated contractions is referred to as retraction. Retraction gradually reduces uterine cavity volume, contributing to the descent of the presenting fetal part, the effacement of the cervical canal, and the dilation of the cervix.
Abdominal Wall and Diaphragm Contraction Force
Contractions of the abdominal wall and diaphragm act as essential auxiliary forces during the second stage of labor, aiding in the expulsion of the fetus. Once the cervix is fully dilated, uterine contractions, combined with the pressure exerted by the amniotic sac or the presenting fetal part on the pelvic floor and rectum, reflexively induce a pushing motion similar to defecation. The mother actively holds her breath and pushes downward, engaging the abdominal wall and diaphragm muscles to produce strong contractions that increase intra-abdominal pressure. This pressure is most effective when synchronized with uterine contractions during the late second stage, facilitating fetal delivery. It can also aid in the expulsion of the detached placenta during the third stage. Premature or excessive use of abdominal pressure can result in maternal fatigue, cervical edema, and prolonged labor.
Levator Ani Muscle Contraction Force
The contraction force of the levator ani muscle facilitates the internal rotation of the presenting fetal part within the pelvic cavity. When the occiput of the fetal head reaches the subpubic arch, this force assists with the extension and delivery of the fetal head. Additionally, when the placenta is already in the vaginal canal, levator ani muscle contractions contribute to its expulsion.
Birth Canal
The birth canal refers to the passage through which the fetus is delivered, consisting of the bony birth canal and the soft birth canal.
Bony Birth Canal
The bony birth canal refers to the true pelvis, which constitutes a significant part of the birth canal. Its size and shape are closely related to the process of childbirth. The pelvic cavity is generally divided into three imaginary planes, commonly referred to as pelvic planes.
Pelvic Inlet Plane
The pelvic inlet plane represents the boundary between the true and false pelvis, forming a transverse oval shape. It involves four diameters: anteroposterior diameter, transverse diameter, left oblique diameter, and right oblique diameter.
- Anteroposterior Diameter (True Conjugate): The distance from the midpoint of the upper border of the pubic symphysis to the midpoint of the sacral promontory, with an average length of approximately 11 cm. This diameter is closely related to the engagement of the fetal presenting part.
- Transverse Diameter: The maximum distance between the iliopubic eminences on both sides, with an average length of approximately 13 cm.
- Oblique Diameter: The left oblique diameter refers to the distance between the left sacroiliac joint and the right iliopubic eminence, while the right oblique diameter is the distance between the right sacroiliac joint and the left iliopubic eminence. The average length of both is approximately 12.75 cm.
Figure 3 Diameters of the pelvic inlet plane
Mid-Plane of the Pelvis
The mid-plane of the pelvis is the narrowest plane of the pelvic cavity, having a longitudinal oval shape. Its size has the most significant impact on childbirth. The anterior portion comprises the lower border of the pubic symphysis, the lateral boundaries are the ischial spines, and the posterior boundary is the lower end of the sacrum. This plane includes two diameters:
- Transverse Diameter (Interspinous Diameter): The distance between the ischial spines on both sides, with an average length of approximately 10 cm. Its length is closely related to the internal rotation of the fetal presenting part.
- Anteroposterior Diameter: The distance from the midpoint of the lower border of the pubic symphysis to the midpoint of the line connecting the ischial spines, which extends to the lower end of the sacrum, with an average length of 11.5 cm.
Figure 4 Diameters of the mid-pelvic plane
Pelvic Outlet Plane
The pelvic outlet plane is composed of two triangular planes:
- The anterior triangle's apex is at the lower border of the pubic symphysis, and its sides consist of the inferior pubic rami.
- The posterior triangle's apex is at the sacrococcygeal joint, and its sides consist of the sacrotuberous ligaments.
Figure 5 Diameters of the pelvic outlet plane (Oblique view)
This plane has four diameters:
- Anteroposterior Diameter: The distance from the lower border of the pubic symphysis to the sacrococcygeal joint, with an average length of approximately 11.5 cm.
- Transverse Diameter (Intertuberous Diameter): The distance between the inner edges of the ischial tuberosities on both sides, with an average length of approximately 9 cm. This diameter is closely related to the passage of the presenting part through the pelvic outlet during delivery.
- Anterior Sagittal Diameter: The distance from the lower border of the pubic symphysis to the midpoint of the transverse diameter, with an average length of approximately 6 cm.
- Posterior Sagittal Diameter: The distance from the sacrococcygeal joint to the midpoint of the transverse diameter, with an average length of approximately 8.5 cm. If the transverse diameter is slightly short, the posterior sagittal diameter should also be measured. When the combined measurement of the transverse and posterior sagittal diameters is greater than 15 cm, a medium-sized, full-term fetal head can pass through the posterior triangle for vaginal delivery.
Pelvic Axis and Pelvic Inclination
The pelvic axis is the curved line connecting the midpoints of each imaginary pelvic plane. During labor and assisted delivery, the fetus moves along this axis. The upper segment of the axis inclines downward and backward, the middle segment inclines directly downward, and the lower segment inclines downward and forward. The pelvic inclination refers to the angle formed between the pelvic inlet plane and the horizontal plane when a woman is in an upright position, which is generally around 60°. An excessive pelvic inclination may affect engagement of the fetal head. Adjusting the maternal position can result in changes to the pelvic inclination.
Figure 6 Pelvic axis
Figure 7 Pelvic inclination
Soft Birth Canal
The soft birth canal consists of a curved passage formed by the lower uterine segment, cervix, vagina, and pelvic floor soft tissues.
Formation of the Lower Uterine Segment
The lower uterine segment develops from the uterine isthmus, which is located between the anatomic internal os and histologic internal os of the cervical canal in its non-pregnant state. During pregnancy, the uterine isthmus measures approximately 1 cm in length but gradually elongates into part of the uterine cavity after 12 weeks of gestation. By the end of pregnancy, the lower uterine segment has fully developed. During labor, regular contractions elongate the lower uterine segment to 7–10 cm. Retraction of muscle fibers in the upper uterine segment results in its wall becoming progressively thicker, while the lower segment becomes thinner due to passive stretching. A circular ridge, the physiological retraction ring, forms at the junction of the upper and lower uterine segments. Under normal conditions, this ring is not externally visible.
Figure 8 Formation of the lower uterine segment and cervical dilation
Figure 9 Physiological retraction ring
Effacement of the Cervical Canal and Dilatation of the Cervical Os
Two key changes occur in the cervix during labor: effacement of the cervical canal and dilatation of the cervical os. In primiparous women, effacement typically occurs first, followed by dilation. Effacement and dilation are primarily the result of uterine contraction and retraction pulling upward. Before labor begins, the cervical canal measures approximately 2–3 cm in length. During labor, contractions and pressure from the presenting fetal part and the forewater cause the internal os to dilate upward and outward, leading to a funnel-like shape. The cervical canal then gradually shortens and ultimately disappears. Contractions also help the fetal presenting part engage, while retained forewaters exert direct pressure on the cervix, facilitating dilation. As full dilation is approached, the membranes often rupture spontaneously, allowing the presenting fetal part to press directly against the cervix, accelerating the dilation process. Only when the cervix is fully dilated can the full-term fetal head pass through. For multiparous women, effacement and dilation tend to occur simultaneously.
Figure 10 Cervical canal obliteration and cervical dilation
Changes in the Vagina, Pelvic Floor, and Perineum
The normal elasticity of the vagina generally does not interfere with childbirth. During labor, the forewaters and fetal presenting part distend the upper vagina. After the membranes rupture, the presenting part directly compresses the pelvic floor, forming a curved tubular passage in the lower portion of the soft canal that projects forward and upward. The vaginal mucosa unfolds its rugae, and the vaginal walls expand and widen. The levator ani muscle spreads downward and outward, with muscle fibers stretching significantly, reducing the perineal thickness from about 5 cm to 2–4 mm to facilitate fetal passage. However, the significant pressure exerted on the perineal body during delivery can result in tearing.
Fetus
The size, fetal position, and presence of abnormalities are key factors influencing labor and determining its level of difficulty. Estimated fetal weight is typically calculated through ultrasound examination combined with fundal height measurements. When the estimated fetal weight differs from the actual birth weight by less than 10%, the estimation is considered fairly accurate. The fetal head is the largest part of the fetus and represents the most challenging portion to pass through the birth canal. Even with a normal pelvic size, an oversized fetus, resulting in an excessively large head diameter, may cause cephalopelvic disproportion and lead to obstructed labor.
Fetal Head Diameters and Fontanelles
Fetal Head Diameters
The fetal head has four main diameters: biparietal diameter, occipitofrontal diameter, suboccipitobregmatic diameter, and occipitomental diameter. The biparietal diameter is commonly used to assess the size of the fetus. Engagement typically occurs through the occipitofrontal diameter, while the suboccipitobregmatic diameter passes through the birth canal. The measurements and lengths of the fetal head diameters are shown in the corresponding tables.
Table 1 Measurements and lengths of fetal head diameters
Fontanelles
The areas where cranial sutures meet and leave larger gaps are called fontanelles. The anterior fontanelle, also called the large fontanelle, is a diamond-shaped space at the junction of the frontal bones, parietal bones, sagittal suture, coronal suture, and frontal suture. The posterior fontanelle, also called the small fontanelle, is a triangular space formed at the junction of the parietal bones, occipital bone, and cranial sutures. The fontanelles serve as important landmarks for determining fetal position during delivery. The cranial sutures and the fontanelles allow cranial bones to have some mobility. As the fetal head passes through the birth canal, it is compressed, and slight overlapping of the cranial sutures occurs, enabling the fetal head to mold and reduce in size, facilitating delivery.
Figure 11 Fetal skull, cranial sutures, fontanelles, and diameters
Fetal Position
The birth canal is a vertical passage. When the fetal longitudinal axis aligns with the pelvic axis in a longitudinal lie (cephalic or breech presentation), it is easier for the fetus to pass through the birth canal. In a cephalic presentation, the fetal head passes through the birth canal first. Through palpation of the sagittal suture and the anterior and posterior fontanelles, the fetal position can be identified. The occiput anterior position is the most favorable for completing the mechanisms of labor and for delivery. Other fetal positions increase the difficulty of delivery to varying degrees.
In a breech presentation, the buttocks are delivered first. Because the circumference of the buttocks is smaller than the head and softer, the birth canal may not fully dilate, leaving no opportunity for head molding when the head is delivered later. This makes the delivery of the head more difficult than that of the buttocks. When the fetus is preterm, the head is relatively larger compared to the buttocks, which increases the risk of after-coming head complications.
In a shoulder presentation, the fetal longitudinal axis is perpendicular to the pelvic axis, making it impossible for a full-term, live fetus to pass through the birth canal. This situation presents a significant risk to both the mother and the fetus.
Fetal Abnormalities
Abnormal fetal development, such as hydrocephalus or conjoined twins, often results in an enlarged fetal head or body, making passage through the birth canal difficult.
Sociopsychological Factors
Although childbirth is a physiological process, it can create significant psychological stress for mothers. Sociopsychological factors can induce physiological changes that may affect uterine contractions and, therefore, the outcome of labor. These factors represent an important determinant of childbirth. Fear and anxiety related to labor pain can lead to weak uterine contractions, slow cervical dilation, fetal head malposition, prolonged labor, and complications such as fetal distress or postpartum hemorrhage.
Providing psychological support to mothers during labor is essential. Detailed explanations of the physiological processes of labor, along with guidance on breathing techniques and physical relaxation, help reduce maternal anxiety and fear. Addressing these psychological concerns enhances the mother's ability to cope with labor and contributes to a more favorable delivery outcome.