This chapter should be cited as follows:
Dutta A, Glob. libr. women's med.,
ISSN: 1756-2228; DOI 10.3843/GLOWM.414323

The Continuous Textbook of Women’s Medicine Series – Obstetrics Module

Volume 11

Labor and delivery

Volume Editor: Dr Edwin Chandraharan, Director Global Academy of Medical Education and Training, London, UK

Chapter

Presentation and Mechanism of Labor

First published: February 2021

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INTRODUCTION

The mechanism of normal labor is series of events that take place in the genital organ that allow the birth of a viable fetus at term; followed by expulsion of placenta and membrane from the vagina.

World Health Organization defines normal labor as starting spontaneously at term (37 completed weeks of gestation) for a fetus with cephalic presentation, progressing without maternal or fetal complication, and resulting in the delivery of fetus followed by placenta and membranes.

The factors that trigger labor at term are not clearly understood; it is postulated that it is a result of changes in the hypothalamic–pituitary–adrenal axis, increasing fetal cortisol, and placental enzymatic functions. Complex interactions of hormones between uterus, placenta and fetus. Fetal dehyroepiandrosterone sulfate (DHEAS) is converted to estriol and estradiol by the placenta.¹ This potentiates oxytocin receptors in the myometrium, reduces the progesterone/estrogen ratio and upregulates myometrial gap junctions to facilitate uterine contractions. The onset of labor is also associated with an increase in prostaglandin production in the placental and cervix, furthering inducing their receptors and facilitating cervical ripening (PGE₂) and uterine contractions (PGF_2a).^2,3,4

MECHANISM OF NORMAL LABOR

For a successful normal labor a coordinated interaction of the uterine activity (power), maternal pelvis (passage) and fetus (passenger) is required.

Maternal pelvis (passage) 

The maternal pelvis is made of five bones (Figure 1): the sacrum and coccyx posteriorly, two innominate bones on each side, and the pubic bone anteriorly. The bones are articulated together by four joints: anteriorly symphysis pubis, two sacroiliac joints posteriorly and the sacrococcygeal joint inferiorly.

The pelvic brim extends from the sacral promontory, along the ilium on each side circularly along the ridge divides the pelvis into upper false pelvis and lower true pelvis.

The significance of the false pelvis is to support the pregnant uterus; the true pelvis is a bony passage for fetus to pass during labor.

The true pelvis is shallow anteriorly, formed by the symphysis pubis (4–5 cm), and deep posteriorly, formed by the sacrum and coccyx (10 cm). It is divided into three parts – inlet, cavity and outlet (Figure 1).

The pelvic inlet has a wide transverse diameter – approximately 13 cm, the midcavity of the pelvis is round, whilst the outlet has a wide anterior posterior diameter.

Uterine activity (power)

The uterine contraction is characterized by its intensity, frequency, and duration. External tocodynamometry is a qualitative measurement of uterine activity, records uterine activity and correlates fetal heart rate (FHR) pattern with uterine contraction.

Quantitative assessment of intrauterine pressure to measure the strength of uterine contraction is done by placement of an intrauterine catheter. This is measured in Montevideo units (MVU). Uterine activity varies in different stages of labor: latent phase approximately 100 MVUs, active phase of labor 175 MVUs and 250 MVUs during the second stage.^5,6

Fetus (passenger)

For a successful outcome, the fetal skull, shoulders, trunk and buttocks should pass through maternal pelvis.

Several variables in the fetus influence its journey through the birth canal.

Fetal size can be estimated by palpation, ultrasound scan and customized growth chart but all of these methods are subjected to large degree of error.

Fetal lie is the relationship of the long axis of the fetus relative to longitudinal axis of the uterus. A fetus in longitudinal lie is suitable for vaginal delivery.

Presentation – the part of the fetus that directly overlies the lower pole of the uterus/pelvic inlet. Hence in longitudinal lie the fetus may be cephalic or breech and in oblique/transverse shoulders or compound with more than one part overlying the pelvic inlet.

Attitude – position of fetal head with the fetal spine (the degree of flexion and/or extension of the fetal head. Flexion of fetal head is a favored attitude as it presents the smallest diameter to the maternal pelvis (Figures 2–5).

Position is the relationship of the presenting part to the maternal pelvis.

Stages of labor: labor is describes in three stages:

• First stage – onset of regular uterine contractions, progressive effacement and dilatation of the cervix to 10 cm. This stage is divided into latent and active phase.
  The duration of the latent phase may vary from days to weeks in primiparous women. It is characterized by regular painful uterine contraction, progressive effacement/dilation of the cervix.
  Active phase is when the cervical dilatation is 4 cm and beyond, in presence of regular painful uterine contraction.
  For a nullipara the first stage of labor this lasts on average 8–18 hours; in multiparous women it is between 5 and 12 hours.
• Second stage – the stage in labor from full dilatation of the cervix to delivery of the baby is defined as second stage.
  Initial second stage is termed as passive second stage: when there is no voluntary maternal effort.
  Active second stage is when there is active maternal effort/expulsive uterine contraction to progressively move the presenting part to deliver the baby.
  Birth is expected within 3 hours of the start of active second stage in most nulliparous and within 2 hours in most multiparous women.⁷
• Third stage – this is the time from the birth of baby till expulsion of placenta and membranes. This is usually completed with 30 minutes of birth following active management or 60 minutes if physiological.

Mechanism of normal labor

The fetus undertakes a series of movements to adapt the smallest possible diameter of the presenting part to the anatomy of the maternal pelvis. The commonest situation is fetus in longitudinal lie, cephalic position and well-flexed attitude.

For description, head is only the index, the trunk also participates in and probably also initiates some movements. These movements are:

• Engagement;
• Descent;
• Flexion;
• Internal rotation;
• Extension;
• Restitution;
• External rotation.

• Engagement is the mechanism by which the greatest transverse diameter of the fetal head: the biparietal diameter (BPD) (9.4 cm) is at or has passed the pelvic inlet (brim). In nulliparous women engagement occurs weeks prior to onset of labor, whereas in multiparous women it may occur in labor.
• Descent is a continuous process throughout the first and second stage of labor.
• Flexion – the head is already flexed to an extent at the time of engagement and further flexion occurs during the first stage of labor due to soft tissue resistance of the pelvis.
• The flexion facilitates the shortest anterior–posterior diameter suboccipito–bregmatic (9.5 cm) to be presented at the pelvic outlet.
• Internal rotation is defined as turning of the head in such a manner that the occiput gradually moves anteriorly towards the symphysis pubis. This carries the long diameter of the head into the antero–posterior diameter (A-P), i.e. the longest diameter of the pelvic outlet from the previous occipito lateral positions.
• Internal rotation brings the occiput forwards under the pubic arch. The fetal shoulder enters the pelvis in the transverse diameter. This results in degree of rotation at the fetal neck.
• Extension (Figure 6) – the force of uterine contraction and active maternal effort along with the pelvic floor muscles facilitates the birth of head by extension. The chin slides over the edge of the perineum and becomes separated from the chest wall, i.e. the head becomes extended. The vaginal outlet is stretched and crowning occurs. With progressive distension of the perineum the occiput, forehead, mouth and chin are delivered successively.
• Restitution (Figure 7) – the visible external movement of the fetal head that corrects the torsion of neck sustained during internal rotation. The direction of movement is opposite to that of the internal rotation (45°).
• This allows the head to come back in line with the shoulders. The occiput points to the maternal thigh of the corresponding side to which it originally lies.
• External rotation (Figure 8) – the movement of the head due to the internal rotation of the shoulder as it comes in the antero–posterior diameter of the pelvic outlet. This is visible externally in a direction opposite to internal rotation. It occurs in the same direction as restitution. Now the shoulders are in antero–posterior diameter (A-P) axis. The anterior shoulder escapes under the pubic arch, while the posterior shoulder sweeps over the perineum.
• After the delivery of the shoulders, the rest of body is delivered spontaneously by lateral flexion.

CONCLUSION

Labor is a crucial time for the mother, family member and the fetus. This is the most perilous journey under taken by the fetus in utero. For the clinician it is equally important to know and identify any deviation from the normal pathway.

Despite immense development in imaging techniques to assist in making the right decision for the patient nonetheless in labor management the clinical assessment still has a key role.

PRACTICE RECOMMENDATIONS

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