This chapter should be cited as follows:
Krishna A, Golob T, et al, Glob. libr. women's med.,
ISSN: 1756-2228; DOI 10.3843/GLOWM.413053

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

The Biology of Parturition: Pelvic Anatomy

First published: February 2021

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INTRODUCTION

The process of parturition requires the successful passage of the fetus through the pelvis aided by uterine contractions and maternal effort. The fetus is often referred to as the passenger, the uterine contractions as the powers and the pelvic anatomy as the passage. The focus of this chapter is the passage itself, both in terms of anatomy and in its clinical application.

THE BONY PELVIS

The pelvis can be conceived as a bounded space with an inlet, walls and outlet. The pelvic inlet represents the point of transition between the abdominal and pelvic cavities. The pelvic inlet is bounded by the sacral promontory posteriorly and the alae (wings) of the sacrum on either side. 

In the pregnant patient the function of the pelvis is to provide support and protection of the gravid uterus, and to permit passage of the fetus to the perineum. It provides attachment for the muscles and membranes that constitute the pelvic floor that ultimately guides the presenting part through the lesser pelvis. It is formed by the fusion of three bones: the ileum, the ischium and the pubis. The ischium may be divided into the ischial body, contributing to a portion of the hip socket or acetabulum, and the ramus, which creates the obturator foramen. The small posteromedial projection from the junction of the body with the ramus is the ischial spine. This has obstetric relevance in both estimating station of the presenting part and providing the landmark for which to perform pudendal nerve anesthesia. If the presenting part is deemed to be at the level of the ischial spines, then it is said to be at midcavity. The bilateral pubic bones are fused in the midline and inferiorly with the ischial rami create the pubic arch. In the female pelvis the angle of the arch is more obtuse but may still vary considerably between individuals with some pubic arches more favorable for accommodating passage of the fetus compared to others. Indeed, in certain areas, assessment of the pubic arch by measuring the distance between the ischial tuberosities is encompassed in vaginal examination in labor. If the distance is great enough to accommodate three fingers side by side, the subpubic angle is considered sufficiently wide.

The pelvic inlet is an oblique plane defined by the pelvic brim. It is formed anteriorly by the superior edge of the pubic symphysis, the pectineal line and pubic crest of the pubic bone, the arcuate line on the inner surface of the ilium and the ala of the sacrum and promontory. It lies at a 60° angle with respect to the horizontal plane. The obstetric conjugate is the distance between the sacral promontory and the inferior margin of the pubic symphysis, i.e. the anteroposterior diameter of the pelvic inlet. This should be more than 11 cm. It can be measured with the examining hand by placing the tip of the index finger on the promontory and the index finger on the pubic bone. This is the narrowest fixed distance the fetal head must pass on entering the lesser pelvis. The transverse diameter at this level is usually approximately 13.5 cm and, therefore, the fetal head usually enters the pelvis in a transverse position. At term in pregnancy, the pelvic ligaments at the sacroiliac joints and pubic symphysis relax under the influence of hormones such a relaxin resulting in increased movement at the pelvic joints and may increase the circumference of the pelvic inlet by 10–15%. Patients experience this ligamental laxity as pain which is particularly aggravated by weight bearing and rotational forces, i.e. turning over in bed. The obstetric conjugate is a fixed distance and remains unaltered.

The pelvic outlet is bound by the pubic arch anteriorly, the ischial tuberosities just laterally, the inferior margins of the sacrotuberous ligament heading posteriorly and the tip of the coccyx in the midline posteriorly. Here the diameters are reversed. The transverse diameter is 11 cm and the anteroposterior diameter is 13.5 cm. Therefore, the fetal head must rotate by 90° into either the occipitoanterior or occipitoposterior position in order to be delivered to the perineum.

THE PELVIC FLOOR

The area between the inlet and outlet is known as the lesser pelvis which is of major obstetric significance.

The floor of the pelvis is formed by the pelvic diaphragm, a group of muscles that collectively form a funnel shape to support the organs of the lesser pelvis and separate them from the perineum. The two primary muscles are the levator ani and the coccygeus muscle. The levator ani muscle is a broad sheet of muscles composed of the pubococcygeus muscle and iliococcygeus. The pubococcygeus can be further subdivided into the pubovaginalis, puboperinealis, puborectalis and puboanalis depending on the insertion of its terminal fibers. The urogenital hiatus, a gap between the anteriomedial borders of the levator ani group permits the vagina and urethra. During parturition, the pelvic floor is supporting the fetal head, whilst the cervix is dilating to permit passage. During this time the pelvic diaphragm may be injured/torn. These muscles are important as they provide support to the urethra, vagina and rectum. Damage may result in prolapse or incontinence of stool or urine.

The inferior one-fifth of the vagina receives somatic sensory innervation from the deep perineal nerve, a branch of the pudendal nerve. Nerves to the upper vagina are visceral and derive from the uterovaginal nerve plexus which supplies the pelvic viscera from the inferior hypogastric plexus.

Pain from the uterine fundus and body follow the sympathetic nerve ganglia to reach the inferior thoracic-superior lumbar spinal ganglia. Afferent fibers from cervix and vagina follow the parasympathetic fibers through the uterovaginal and inferior hypogastric plexus and pelvic splanchnic nerves to reach cell bodies in the spinal sensory ganglia of S2–4.

Anatomy of childbirth, the bony pelvis

The fetal skull is made mainly of thin pliable bones forming the vault. This is anchored to the rigid and incompressible bones at the base of the skull. The fetal skull is the most difficult part of the baby to pass through the mother’s pelvic canal, due to the third bony nature of the skull. Understanding the anatomy of the fetal skull and its diameters will help in recognizing how parturition is progressing and to understand how the fetal head is presenting in relation to the birth canal.

Areas of skull

The skull is arbitrarily divided into several zones of obstetrical importance:

• The vertex is a quadrangular area bounded anteriorly by the bregma and coronal sutures, behind by the lambda and lambdoid sutures and laterally by the lines passing through the parietal eminences. This is the normal and the safest presentation for a vaginal delivery.
• The brow is an area bounded on one side by the anterior fontanelle and coronal sutures, and on the other side by the root of the nose and supraorbital ridges of either side. A brow presentation is a significant risk for the mother and the baby.
• The face is an area bounded on one side by root of the nose and supraorbital ridges and, on the other, by the junction of the floor of the mouth with neck. A face presentation is also a significant risk for the mother and the baby.

Diameters of the skull

The engaging diameter of the fetal skull depends on the degree of flexion present. The anteroposterior diameters of the head which may engage are shown below (Table 1).

The transverse diameters which are concerned in the mechanism of labor are:

• Biparietal diameter (9.5 cm) extends between two parietal eminences. Whatever may be the position of the head, this diameter nearly always engages.
• Super-subparietal (8.5 cm) extends from a point placed below one parietal eminence to a point placed above the other parietal eminence of the opposite side.
• Bi-temporal diameter (8 cm) is the distance between the antero-inferior ends of the coronal suture.
• Bi-mastoid diameter (7.5 cm) is the distance between the tips of the mastoid processes.

The mechanism of labor has been extensively described in another chapter. For the purpose of describing the mechanics of parturition some of the commonly occurring positions which include the occipitoanterior, occipitoposterior and occipitotransverse positions are briefly described.

Occipitoposterior position

It is a vertex presentation where the occiput is placed posteriorly over the sacroiliac joint or directly over the sacrum.

When the occiput is placed over the right sacroiliac joint the position is called right occipitoposterior (ROP) and when placed over the left sacroiliac joint, it is called left occipitoposterior (LOP) and when it points towards the sacrum, is called direct occipitoposterior.

ROP is more common than the LOP due to dextro-rotation of the uterus and the presence of the sigmoid colon on the left. The prevalence of the OP position is 15–32% at the onset of labor,^1,2,3,4 10–20% early in the second stage of labor and 5–8% at delivery.^3,5,6,7

Mechanism of labor

The head engages through the right oblique diameter in ROP and left oblique diameter in LOP. The engaging transverse diameter of the head is biparietal diameter (9.5 cm) and the anteroposterior diameter is either suboccipito-frontal (10 cm) or occipito-frontal (11.5 cm). Because of deflexion, engagement is delayed.

In favorable circumstances:

• Flexion: Good uterine contractions result in good flexion of the head. Descent occurs until the head reaches the pelvic floor.
• Internal rotation of the head: As the occiput is the leading part, it rotates 3/8th of a circle anteriorly to lie behind the symphysis pubis. As the neck cannot sustain such amount of torsion, the shoulders rotate about 2/8th of a circle to occupy the right oblique diameter in ROP and the left oblique diameter in LOP with 1/8th of a circle torsion of the neck still left behind. Thus, the rest of the mechanism is like that of right occipitoanterior in ROP and that of left occipitoanterior in LOP.

In unfavorable circumstances

• Sometimes the occiput fails to rotate as described previously.
• Incomplete forward rotation: The occiput rotates through 1/8th of a circle anteriorly and the sagittal suture comes to lie in the bispinous diameter. This position is called deep transverse arrest.
• Non-rotation: Both the sinciput and the occiput touch the pelvic floor simultaneously due to moderate deflexion of the head resulting in non-rotation of the occiput. The sagittal suture lies in the oblique diameter. Further rotation is unlikely, and the condition is called oblique posterior arrest.
• Malrotation: In extreme deflexion, the sinciput touches the pelvic floor first resulting in anterior rotation of the sinciput to 1/8th of a circle and putting the occiput to the sacral hollow. This is called persistent occipitoposterior position.

Anatomy of childbirth, pelvic musculature

There have been classically four different 'types' of female pelvis described in medical literature⁸ gynecoid, android, anthropoid and platypelloid listed here in order of frequency. These different anatomical pelvises are described in detail above and have implications for the passage of the fetus through the pelvis. The gynaecoid pelvis is the most common anatomical arrangement and typically the fetal head will enter the pelvis in a transverse position later rotating to an anterior or less commonly posterior orientation. In contrast, the anthropoid pelvis more commonly sees initial engagement of the fetal head in an anterior or posterior position, whilst with the platypelloid pelvis the transverse orientation is almost always found due to the narrow anteroposterior diameter and will often persist until later than with other pelvic types. According to this description there are marked variations of these pelvic types amongst different ethnic groups with the anthropoid pelvis more common in Afro-Caribbean women. It has been observed that as obstetric literature has its modern origin in Europe, this has created a Eurocentric view where the classically described stages of fetal passage through the pelvis reflect the most common process typically observed in European women.⁹ This has clinical relevance as it has been argued by authors that in the past higher numbers of rotational instrumental deliveries, with associated morbidity were undertaken in African-American women due to the later persistence of what were thought to be malpositions.¹⁰

Late in pregnancy, the presenting fetal part, usually the head, is normally found to be 'engaged', indicating it has passed below the level of the pelvic inlet. Following abdominal palpation, the presenting part is referred to as being palpable in fifths indicating that the remainder has entered the true pelvis, 0/5 palpable meaning a head that is no longer in the abdominal cavity and is completely engaged in the pelvis. This, of course, has clinical relevance during labor as the presenting part will be observed to progressively enter the pelvis. If this process does not occur, it can be an early sign of obstruction or inadequate progress due to relative or true cephalo pelvic disproportion. Similarly, during the second stage of labor decision on the safety of attempting instrumental delivery will be guided by both the position and the station of the fetal head with a station corresponding to no more than 1/5 palpable abdominally now usually agreed to be a prerequisite for safe instrumental delivery.¹¹

The different diameters of the bony pelvis and the various types of pelvic shape are considered above, here we turn our attention to the role of the pelvic muscles. The pelvic muscles are crucial in aiding the passage of the fetus through the bony pelvis. The pelvic diaphragm naturally encourages the fetus to adopt a well-flexed vertex presentation, relaxation of the pelvic muscles through neuraxial blockade can interfere with expulsive efforts in the second stage of labor and, therefore, contribute to the increased rate of instrumental delivery observed in women with epidural anesthesia.¹²

The lateral walls of the pelvis are formed by the obturator internus and piriformis muscles, which help to both abduct and externally rotate the hip joint. Ligaments travel between the sacrum and ischial spines (sacrospinous ligaments) and sacrum and ischial tuberosity (sacro tuberous ligament) forming two windows (foramen) in each lateral hemipelvis. The muscles of the lateral wall complete this effect bounding the greater sciatic foramen (above and below the piriformis muscle) and the lesser sciatic foramen (medial to obturator internus muscle). The pelvic diaphragm hangs across the bony pelvis just above the level of the lesser sciatic foramen such that the level of the lesser sciatic foramen can be considered to be within the perineum rather than pelvis. Various neurovascular structures pass through these spaces, the most clinically significant of these being the sciatic nerve, pudendal nerve and artery. The sciatic nerve formed from the roots of L4-S3 travels over the anterior surface of piriformis to exit the pelvis and travel down the posterior surface of the leg. The pudendal nerve originating from the ventral rami of S2-S4 exits the pelvis through the greater sciatic foramen and then tracks around the sacrospinous ligament to pass back through the lesser sciatic foramen, close to the ischial spine, returning to the pelvis to innervate the perineum. The pudendal nerve can, therefore, be utilized clinically to provide perineal anesthesia by palpating the ischial spines. Local anesthetic can be infiltrated medial to this infiltrating, the lateral portion of the sacrospinous ligament.

The muscular floor of the pelvis is formed of a pelvic diaphragm consisting of the levator ani and coccygeus muscles. Anteriorly the muscles of the levator ani converge on the perineal membrane and caudally the muscles of the deep perineal pouch. The muscles of the pelvic diaphragm form a hammock structure that acts like a funnel guiding the fetus through the birth canal. The muscles are attached superiorly to the walls of the pelvis. The pelvic diaphragm separates the pelvic cavity above from the perineum below. These muscles give support to the pelvic organs and as such damage during childbirth can predispose a woman to pelvic organ prolapse later in life. Whilst injuries to the superficial perineal muscles will usually be apparent on external examination, avulsion of the levator ani muscles from their superior attachments is not usually detectable by clinical examination but can be visualized by various other imaging modalities. Due to the higher forces generated by traction and shortening of the second stage, it is assumed by many that levator avulsion is more commonly associated with instrumental deliveries and, consequently, these confer a greater life time risk of pelvic organ prolapse.¹³

Below the levator ani muscles lie the muscles of the perineum. The perineal membrane is a fascial structure that covers the pelvic outlet. It is punctured by the urethra and vagina anteriorly and the rectum posteriorly. Just above the perineal membrane in the deep perineal pouch are found the deep transverse perineal muscles, the sphincter urethrovaginalis and the compressor urethrae. Below the perineal membrane are situated the superficial perineal muscles. The muscles of the pelvic floor and perineum are linked by the perineal body, a conglomeration of connective tissue formed from the deep pelvic muscles and muscles of the perineal region. Connecting to the perineal body are posteriorly the muscles of the external anal sphincter, the deep and superficial transverse perineal muscles in the midline and anteriorly the bulbospongiosus muscles. During vaginal delivery, the perineal body is frequently disrupted with damage to these surrounding muscles. The perineal body divides the perineal muscles anteriorly from the anal sphincter complex posteriorly. A shorter length of the perineal body is therefore associated with increased risk of anal sphincter injury. Active 'hands on' delivery of the fetus during vaginal birth aims to support and guard the perineal body and has an evidence base for reducing injury to the anal sphincter complex.¹⁴ A universally recognized system for classifying perineal trauma defines 1st degree trauma as injury to the vaginal mucosa, 2nd degree trauma involving the muscles of the perineum but not the anal sphincter complex, and 3rd and 4th degree tears involving damage to the anal sphincter complex. Within this classification 3rd degree injuries are further divided into 3a meaning less than 50% of the external sphincter thickness has been torn, 3b more than 50% of external sphincter but intact internal sphincter, and 3c injuries meaning there is also injury to the internal sphincter but intact rectal mucosa. Fourth degree anal sphincter injuries mean damage to the level of the anal epithelium with complete disruption of the internal and external anal sphincters.¹⁵ It is possible to sustain injuries above the level of the sphincter complex that enter the rectum through the posterior wall of the vagina. These “button hole tears” can occur with intact anal sphincters and are at risk of being missed without thorough perineal and per rectal exam after delivery. Mediolateral episiotomy involves incision through the bulbospongiosus muscle, superficial transverse perineal muscle, perineal body and is an iatrogenic 2nd degree injury. This is ordinarily done at a 60° angle from the midline to decrease the risk of anal sphincter injury. The most recent Cochrane reviews do not support routine episiotomy as a technique for reducing anal sphincter injury in normal vaginal birth, but it is usually recommended during instrumental delivery particularly with nulliparous women if forceps are used.¹⁶ The rationale behind episiotomy in this instance is in the case of forceps to widen the soft tissue dimensions of the vagina creating room for the instrument, also in instrumental delivery the artificial shortening of the delivery probably gives less time for tissues to naturally stretch making uncontrolled tears more likely. Episiotomy can also be undertaken to expedite delivery, prevent complex tears or enable internal maneuvers in situations such as shoulder dystocia or breech presentation. Various techniques of episiotomy have been described, with lateral, mediolateral and medial (midline) being the most common. An anterior episiotomy, also known as de-infibulation is almost exclusively used in women with type 3 female genital mutilation (FGM). This is described elsewhere in this series. The mediolateral episiotomy is most widely practised in Europe and commonwealth countries, and involves a diagonal incision made from the introitus though the bulbospongiosus and superficial transverse perineal muscles. This incision generally results in greater blood loss but is cosmetically easy to repair. 

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