14 Labour
Labour
Features of labour
Mechanical features of labour
There are three mechanical features of labour commonly described:
1. The powers: this is the uterine force of contractions that will expel the fetus. In labour an efficient uterus will contract regularly for 40–60 seconds every 2–3 minutes. These contractions will cause descent of the presenting fetal part, allowing effacement and subsequent dilatation of the cervix. By definition, the nulliparous uterus is inefficient, in that the uterus often contracts irregularly, as opposed to the multiparous uterus.
2. The passage: the bony pelvic canal has to be traversed by the fetus in labour and, therefore, the diameters of the pelvis need consideration (Figs 14.1 and 14.2). The pelvic inlet has two diameters:
Figure 14.1 Diameters of the pelvis, the pelvic brim.
(Redrawn with permission from Rymer J, Davis G, Rodin A et al. 2003 Preparation and Revision for the DRCOG, 3rd edn. Edinburgh: Churchill Livingstone, p. 382.)
3. The passenger is the fetus and the fetal head diameters (Fig. 14.3) are important to allow ease of descent:
Mechanical features of the vertex in labour
The mechanics of labour described above cause changes in the attitude of the fetal head during descent into the pelvis (Fig. 14.4) and are summarized in stages as:
1. Descent: the fetal head often descends during the last few weeks of pregnancy in the nulliparous patient, but this descent may not occur until just before the onset of labour in multiparous patients or also in Afro-Caribbean women because of the shape of their pelvises.
2. Engagement: this occurs when the maximum diameter of the fetal head has passed through the pelvic inlet. It is usually described in fifths of the fetal head that can be palpated abdominally, with engagement being defined as less than or equal to two-fifths of the fetal head palpable above the pelvic brim.
3. Flexion: the fetal head flexes as it descends through the pelvis in order to allow its smallest diameter to traverse the pelvis. With maximum flexion the posterior fontanelle can be easily palpated vaginally.
4. Internal rotation: as the fetal head descends it rotates from a LOT position to OA, as described above (Fig. 14.5).
5. Extension: the fetal head remains flexed until it reaches the perineum and then begins to extend as the head crowns as it is about to be delivered.
6. External rotation (restitution): as the fetal head is delivered it reverts to the transverse position (OT).
7. Lateral flexion: when the head is delivering the anterior shoulder appears, followed by the posterior shoulder with the trunk delivering by lateral flexion.
Assessment of labour
1. History: the history on admission should include specific questioning on the following:
Management
Once the diagnosis of labour has been made then the membranes can be ruptured to assess the liquor for the presence of meconium, which will provide information on fetal well-being. A partogram is a visual record of labour progress and records a variable amount of information depending on each individual obstetric unit. The following is a list of usual information recorded in a partogram (Fig. 14.6):
2. The date and time of admission
4. A progressive representation of cervical dilatation (descent of the head). The partogram may include an action line denoting a minimum acceptable rate of cervical dilatation (usually 1 cm/h for a primigravid patient). If progress falls below the action line, then medical intervention may be considered
5. Details on the contractions should include frequency (usually described as the number occurring in 10 minutes), duration and strength
6. The fetal heart rate by intermittent auscultation or continuous fetal monitoring
7. Colour of liquor and time of rupture of membranes
8. Drugs given, including oxytocin and analgesics
9. Maternal urine production and urinalysis for ketones and/or protein.
Progress of labour
Primiparous labour
Labour should proceed at a rate of at least 1 cm cervical dilatation per hour in a primiparous patient. The first stage should, therefore, last for 10 hours with 2 hours for the second stage. Labour that lasts for longer than 12 hours is said to be prolonged. However, the primiparous uterus is notoriously inefficient as it has not experienced labour before, so that even if the contractions feel strong and frequent they may be having little effect on cervical dilatation. This is correctable if diagnosed early by performing an amniotomy if the membranes are still intact and by commencing an oxytocin infusion to augment labour. Careful clinical monitoring is required to ensure that contractions do not exceed one every 2 minutes or hypertonia results, with fetal hypoxia caused by restriction of the maternal placental blood flow. Oxytocin is, therefore, administered by titrating the infusion against the uterine contractions to achieve efficient uterine action and each obstetric unit will have protocols for concentration and infusion rates (Fig. 14.6). If full dilatation is not foreseeable within 12 hours, then caesarean section may need to be considered as the woman will become exhausted and there will be increasing risk of fetal hypoxia. The partogram is a crucial guide to labour progression. An individual decision is taken by each woman on the recommendation of the obstetrician and midwife.
Fetal surveillance in labour
Methods
1. Amniotic fluid and liquor: the absence of liquor is associated with intrauterine growth retardation. Meconium is the passage of stool by the fetus in utero and is caused by vagal stimulation of the gut as a result of an episode of hypoxia. Some units will grade the meconium during labour from 1 to 3. Grade 1 meconium is lightly stained fluid (meconium that is very diluted by adequate liquor), grade 2 is moderately stained and grade 3 is a heavy, thick liquid associated with the fresh passage of meconium in the presence of reduced liquor, which is of particular concern, indicating fetal hypoxia.
2. Intermittent auscultation with either a Pinard’s stethoscope or a Sonicaid. This is sometimes the method of choice in monitoring a low-risk labour and should be performed for 1 minute after a contraction and every 15 minutes to detect accelerations or decelerations of the fetal heart rate.
3. Continuous fetal monitoring using either an abdominal transducer (as part of the CTG) or a fetal scalp electrode. This will give a continuous real-time record of the fetal heart rate response to labour. The CTG should always be interpreted as part of the whole clinical picture, including stage of labour, maternal analgesia and status of the uterine contractions.
4. Fetal blood sampling is performed when the CTG suggests fetal compromise in labour and is interpreted on the basis of the fetal blood pH. A pH of greater than 7.25 is considered normal; a pH of between 7.20 and 7.25 is a borderline result that will need to be repeated within 1 hour; and a pH of less than 7.20 is abnormal, indicating immediate delivery.
Fetal heart rate patterns
There are three features of the fetal heart rate that should be noted – the baseline, beat-to-beat variability and the presence of accelerations or decelerations (see Fig. 14.7 for examples of CTG traces):
• Normal: the normal baseline should be between 110 and 160 beats per minute (bpm). Baseline beat-to-beat variability should be 5–25 bpm. Accelerations from baseline of 10–15 bpm will indicate a healthy fetus.
• Baseline tachycardia: a baseline fetal heart rate of greater than 160 bpm can be associated with prematurity, hypoxia, maternal pyrexia or drugs and fetal distress if also in the presence of any other abnormality.
• Baseline bradycardia: a baseline fetal heart rate of less than 110 bpm may be associated with postmaturity, hypoxia or heart block. A baseline of less than 90 bpm is usually indicative of impending fetal demise.
• Baseline variability: variability is the presence of irregular accelerations from baseline, and loss of beat-to-beat variability can indicate a sleep pattern if it lasts for less than 40 minutes. If it is prolonged beyond this it indicates hypoxia. Increase in variability may also indicate acute hypoxia.
• Early decelerations: these occur with the onset of a contraction and the fetal heart rate returns to baseline by the end of the contraction. These are usually caused by fetal head compression during descent and are benign.
• Late decelerations: these occur when the fetal heart rate drops after the peak of the contraction, with slow recovery to baseline after the contraction. Late decelerations are associated with fetal hypoxia.
• Variable decelerations: these decelerations, as the name suggests, occur at variable times during labour, often with no association to the contraction, and usually indicate cord compression.
