11: Normal labour and delivery

Normal labour and delivery

11.1 Normal labour and delivery

Anatomy of the female pelvis

The female pelvis is generally wider and broader than that of males, with a more pronounced oval‐shaped inlet (Figure 11.1). This ‘gynaecoid’ pelvis is ultimately different to accommodate the process of childbirth. In obstetric terms, the female pelvis is considered as two regions: the superior section is referred to as the greater or ‘false’ pelvis. It has limited obstetric relevance but provides support for the lower abdominal viscera. Intuitively, the inferior section is known as the lesser or ‘true’ pelvis, which contains the pelvic cavity and pelvic viscera.

Anatomy of a female pelvis with parts labeled pelvic outlet, symphysis pubis, pelvic inlet, sacrum, and ‘false’ pelvis.

Figure 11.1 Anatomy of the female pelvis

Pelvic inlet

The junction between the greater and lesser pelvis is known as the pelvic inlet. The outer bony edges of the pelvic inlet are called the pelvic brim. This is the superior margin of the pelvic cavity and is bounded by the sacral promontory posteriorly, laterally by the iliopectineal lines, and anteriorly by the symphysis pubis. The pelvic inlet determines the size and shape of the birth canal. Significantly, the transverse diameter tends to be greater than the anteroposterior (AP) diameter.

Pelvic cavity

The pelvic cavity has a curved shape because of the difference in size between the anterior and posterior borders of the space created by the pelvic bones. Roughly circular in shape with the transverse and AP diameters tending to be similar, it is bounded by the sacrum posteriorly, laterally by the pubic bone and the obturator fascia and the inner aspect of the ischial bone, and anteriorly by the symphysis pubis. The cavity is referred to as the pelvic canal and is the bony passage through which the baby must pass.

Pelvic outlet

This is the narrowest bony part of the pelvis for the baby to pass through and marks the inferior margin of the pelvic cavity. It is bounded posteriorly by the coccyx, laterally by the ischial tuberosities, and anteriorly by the pubic arch. In regard to the mechanism of normal delivery it is important to note that now the AP diameter tends to be greater than the transverse diameter.

11.2 Fetal skull

Figure 11.2 identifies of certain regions and landmarks of the fetal skull. These have particular importance for obstetric care because they may indicate the presenting part of the fetus.

  • The face extends from the chin to the supraorbital ridge. A face presentation carries a significant risk for both mother and baby (see Chapter 12)
  • The brow extends from the supraorbital ridge to the anterior fontanelle. A brow presentation also carries a significant risk for both mother and baby
  • The vertex is the area midway between the anterior fontanelle, the two parietal bones and the posterior fontanelle. A vertex presentation occurs when this part of the fetal skull is leading the way. This represents a normal presentation for a vaginal delivery
  • The occiput is the area between the posterior fontanelle and the base of the skull. It is a very unusual presenting part, carrying a significant risk to mother and baby
Anatomy of the fetal skull with labels frontal bone, temporal bone, parietal bone, occipital bone, face, Brow (Sinciput), anterior fontanelle, vertex, posterior fontanelle, and occiput.

Figure 11.2 Anatomy of the fetal skull

Additionally, the following terms are also used as reference points: the sinciput refers to the front of the head whilst the occiput refers to the back of the head and the mentum refers to the chin.

The fetal skull bones are as follows:

  • The frontal bone: the fetal frontal bone consists of two halves that after the age of 8 years fuse to form a single bone
  • The two parietal bones lie on either side, forming the majority of the skull
  • The two temporal bones, one on each side of the head, are closest to the ears
  • The occipital bone forms the back and part of the base of the skull

Sutures are joints between the bones of the skull that permit gliding of one bone over another during moulding of the head. In a vertex presentation the diameter of the head can be reduced to allow easier passage through the birth canal. The sutures begin to harden during childhood, inhibiting movement. This continues into early adulthood until the growing process completes. There are two key landmarks created where the sutures meet, and these are referred to as the fontanelles:

  • The anterior fontanelle (also known as the bregma) is a diamond‐shaped space towards the front of the baby’s head. It is formed by four bones, at the junction of the sagittal, coronal and frontal sutures
  • The posterior fontanelle (or lambda) has a triangular shape, and is found towards the back of the fetal skull. It is formed by three bones at the junction of the lambdoid and sagittal sutures

11.3 Stages of labour

The process of labour is normally divided into three stages as this makes it easier to describe the progress a mother is making. These are explained below.

A very fast labour with a duration of less than 1 hour is referred to as a ‘precipitate labour’.

The amniotic membranes and fluid surround the fetus, protecting it within a sterile environment against ascending bacterial infection.

  • The membranes can rupture at any point before or during labour
  • The amniotic fluid or liquor should be clear and odourless
  • If the liquor is bloodstained, this could indicate an abruption or placenta praevia
  • If the liquor is yellow or green coloured, this could indicate the presence of meconium and may be an indicator of fetal compromise
  • The presence of thick meconium that may contain particulate material is a particular cause for concern

11.4 First stage of labour

The first stage can be further divided into the ‘latent’ and the ‘active’ phases.

Latent phase of labour

This phase of labour can be experienced for several weeks prior to the onset of active labour. Uterine contractions experienced during this phase cause the cervix to thin and soften before it can start to dilate. Contractions are often irregular with a stop/start pattern without becoming stronger or longer. This is normal.

Active phase of labour

  • This is defined as regular uterine contractions causing both cervical effacement and dilatation from 0 to 10 cm
  • The presence of a mucus show, which may be bloodstained, is common during both the latent and active first stages of labour and is an indicator of cervical effacement and/or dilatation
  • Contractions should increase in length, strength and frequency as the first stage progresses
  • The frequency of contractions should be measured over a 10‐minute period
  • In established labour, there are 3–4 contractions in 10 minutes each of which should last for about 1 minute
  • More than five contractions within a 10‐minute period can indicate overstimulation and may suggest placental abruption (see Chapter 9)
  • The duration of the first stage can range from minutes to several hours
Mar 9, 2019 | Posted by in EMERGENCY MEDICINE | Comments Off on 11: Normal labour and delivery
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