Physiological and anatomical changes in childbirth

Chapter 7 Physiological and anatomical changes in childbirth



CHAPTER CONTENTS


The passages 53
Bony pelvis 53

Soft tissues of the female pelvis 54

The passenger 57
Anatomy of the fetal skull 57

The powers 60
Characteristics of myometrial contractions 61

Spread of uterine contractions 61

Myometrial activity in pregnancy and labour 61

Pain of uterine contractions 63

Effect of childbirth on the mother 63
Energy expenditure 63

Body temperature 64

Cardiovascular changes 64

Blood loss during childbirth 64

Gastrointestinal tract 64

Effect of childbirth on the fetus 64

Labour and childbirth is the process whereby the fetus and placenta are expelled from the uterus by coordinated myometrial contractions. The reason why labour starts remains obscure in spite of much research and many theories. When this is elucidated it should be possible to prevent premature labour, with its increased perinatal mortality and morbidity.


For labour to commence two things have to occur: the onset of coordinated uterine contractions and the softening of the uterine cervix. The critical factor for the increase in uterine activity is the rise in intracellular calcium, which activates phosphorylation of myosin and the creation of cross-bridges with actin, which results in the contraction of the myometrial cell. The cervical changes are due to a breakdown in collagen owing to the release of metalloproteinases and an increase in the water content. Prostaglandins and leukotrienes are involved in these physiological changes.


The fetus then has to negotiate the birth canal, propelled by contractions of the uterus. Factors that can delay or prevent this are:


• The shape and size of the bony and soft tissues of the woman’s pelvis (the passages)

• The size of the fetus (the passenger)

• The quality and frequency of the uterine contractions (the powers).


THE PASSAGES



Bony pelvis


The bony pelvis is made up of four bones, the two innominate bones, the sacrum and the coccyx, united at three joints. When a woman stands erect, the pelvis is tilted forward. The pelvic inlet makes an angle of about 55 ° with the horizontal. The angle varies between individuals and between races; for example, black Africans have a lesser angle. An angle of more than 55 ° may make the descent of the fetal head into the pelvis difficult (Fig. 7.1).


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Fig. 7.1 Effect of the inclination of the pelvic brim on the engagement of the fetal head.


The ‘true’ pelvis is bounded by the pubic crest, the iliopectineal line and the sacral promontory. An ‘ideal obstetric’ pelvis is described in Box 7.1 and the brim is shown in Figure 7.2. The true pelvis is cylindrical in shape, with a bluntly curved lower end, and is slightly curved anteriorly. Anteriorly the pubic bones form its boundary, measuring 4.5 cm. Posteriorly, the curve of the sacrum forms its boundary, measuring 12 cm. Laterally its walls narrow slightly distally (Fig. 7.3). The walls are penetrated by the obturator foramen anteriorly and the sciatic foramen laterally, which is divided into two parts by the sacrospinous and sacrotuberous ligaments.



Box 7.1 The ideal obstetric pelvis
































Brim Round or oval transversely
No undue projection of the sacral promontory
Anterior–posterior diameter 12 cm
Transverse diameter 13 cm
The plane of the pelvic inlet not less than 55 °
Cavity Shallow with straight side-walls
No great projection of the ischial spines
Smooth sacral curve
Sacrospinous ligament at least 3.5 cm
Outlet Pubic arch rounded
Subpubic angle > 80 °
Intertuberous diameter at least 10 cm

image

Fig. 7.2 The ‘ideal obstetric’ pelvis: pelvic inlet. A–P = anterior–posterior.


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Fig. 7.3 The pelvic cavity. The right innominate bone has been removed in order to show the extent of the cavity.


For descriptive purposes the true pelvis can be divided into four zones. These are shown in Figure 7.4. The measurements of the zone of the inlet are shown in Figure 7.2. The zone of the cavity is wedge-shaped in profile and almost round in section. It is the most roomy part of the true pelvis, the anterior–posterior diameter measuring 13.5 cm and the transverse diameter 12.5 cm.


image

Fig. 7.4 The zones of the pelvis.


The zone of the midpelvis passes through the apex of the pubic arch, the spines of the ischia, the sacrospinous ligament and the tip of the sacrum. It is the smallest zone and its most important diameter is the ischial–bispinous diameter, which measures 10.5 cm. If this zone is contracted the fetal presenting part may not be able to rotate and may become arrested.


The zone of the outlet (Fig. 7.5) does not usually interfere with the birth unless the pubic rami are narrow, which reduces the intertuberous diameter. In these cases delay may occur and the soft tissues of the perineum may be torn and damaged.


image

Fig. 7.5 The female pelvic outlet viewed from below. A–P = anterior–posterior.


The axis of the birth canal corresponds to the direction the fetal presenting part – usually the head – takes during its passage through the birth canal (Fig. 7.6).


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Fig. 7.6 Inclination of the pelvic brim and the axis of the birth canal. The plane of the pelvic brim makes an angle of about 55 ° with the horizontal in the erect patient, and the plane of the outlet about 10 ° with the horizontal. The axis of the birth canal is angled, the alteration in direction occurring at the pelvic floor.



Factors influencing pelvic shape and size


Minor alterations in pelvic shape have been found in many women receiving routine radiological pelvimetry, but only in a few instances has the abnormality been found to delay the birth. Smaller women tend to have a smaller bony pelvis, but also tend to have smaller babies. More severe alterations occur in populations where rickets or osteomalacia are found, but are uncommon in the developed countries today.



Soft tissues of the female pelvis


These include the uterus, the muscular pelvic floor and the perineum. The anatomical details are described in Chapter 43.



Uterus


The uterus in pregnancy can be divided into three parts:


• The upper uterine segment

• The lower uterine segment

• The uterine cervix.


Upper uterine segment


This portion of the uterus consists of the fundus and that part of the uterus lying above the reflection of the vesico-uterine fold of peritoneum. During pregnancy it undergoes the greatest degree of myometrial hyperplasia and hypertrophy. In labour it provides the strong contractions that push the fetus along the birth canal.



Lower uterine segment


This portion of the uterus lies between the vesico-uterine fold of the peritoneum superiorly and the cervix inferiorly. During pregnancy the upper part of the cervix is incorporated into the lower uterine segment, which stretches to accommodate the fetal presenting part (Fig. 7.7). In late pregnancy, as the upper segment muscle contractions increase in frequency and strength, the lower uterine segment develops more rapidly and is stretched radially to permit the fetal presenting part to descend (Fig. 7.8). In labour the entire cervix becomes incorporated into the stretched lower uterine segment.


image

Fig. 7.7 Formation of the lower uterine segment based on the observations of CP Wendell-Smith. Cone, the condensation of muscle at the junction of the body of the uterus and the cervix; AIO, anatomical internal os; HIO, histological internal os.

Related posts:

  1. Cardiovascular, respiratory, haematological, neurological and gastrointestinal disorders in pregnancy
  2. Antepartum haemorrhage
  3. Endometriosis and adenomyosis
  4. Infections of the genital tract

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Jun 15, 2016 | Posted by in OBSTETRICS | Comments Off on Physiological and anatomical changes in childbirth

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