Labour or rupture of membranes is defined as preterm if it occurs at less than 37 completed weeks’ gestation. Preterm prelabour rupture of membranes (PPROM) is diagnosed when rupture of the membranes is not followed by spontaneous uterine contractions before 37 weeks’ gestation. When this occurs after 37 weeks, prelabour rupture of membranes (PROM) is diagnosed; the period of latency required before the diagnosis is made varies but is usually up to 8 hours.
About 5-10% of deliveries are preterm in the UK, in about a third of cases without rupture of membranes as the initiating event. Prematurity is a major cause of fetal and neonatal morbidity and accounts for the majority of infant deaths in the developed world; 75% of perinatal mortality occurs in preterm babies. Many epidemiological studies have investigated neonatal morbidity and mortality according to birth weight instead of gestation, although there is evidence that the interplay of these two factors is more important than either one alone. For example, at a given gestation, heavier babies have less morbidity and mortality than lighter ones; similarly, at a given birth weight, mature babies do better than immature ones.
Although several risk factors for preterm delivery are recognised, about half of preterm deliveries have no obvious precipitating cause. Known risk factors include a previous history of prematurity; young maternal age; maternal disease (especially infection), surgery or trauma; uterine abnormality; stress; smoking and use of recreational drugs; multiple gestation; placenta abnormality; and fetal disease.
Problems and special considerations
Diagnosis
Careful obstetric assessment is required to establish the diagnosis of PROM, since it is not always obvious. If pooling of amniotic fluid is not seen, vaginal fluid can be tested for insulin-like growth factor binding protein-1 or placental a-microglobulin-1, chemicals present in amniotic fluid. However, these tests have a high false-positive rate and must be interpreted in the context of the woman’s clinical condition. Antibiotics are given to women diagnosed with PPROM. The diagnosis of preterm labour is made according to gestation, the frequency of uterine contractions and changes in cervical dilatation or effacement. In some countries (not routinely in the UK) fetal maturity is assessed by the lecithin-sphingomyelin (LS) ratio, which increases as surfactant production increases and may indicate the likelihood of respiratory distress syndrome.
Maternal problems
Prolonged rupture of membranes may lead to chorioamnionitis with or without systemic features of infection. Thus there may be theoretical risks from regional anaesthesia (see Chapter 139, Sepsis, and Chapter 144, Pyrexia curing labour).
Administration of tocolytic drugs may result in tachycardia, fluid overload and pulmonary oedema (see Chapter 64, Oxytocic and tocolytic drugs). Tachycardia may also be related to maternal sepsis and anxiety; the latter may be considerable because of the mother’s fears for her baby.
Any underlying cause of preterm labour or PROM (such as maternal disease) may have implications for the anaesthetic management.
The best method of delivery is controversial, but the operative delivery rate is higher than for term deliveries. Breech presentation is more common. Classical caesarean section may be required if the lower uterine segment is poorly formed (uncommon after 26 weeks’ gestation), with a greater risk of haemorrhage and implications for future pregnancies.
Neonatal problems
These can be short- or long-term. The immediate problems for the neonate are respiratory distress (occurring in approximately 50-60% of babies at 26-28 weeks, 20-40% at 30-32 weeks and 5-10% at 34-36 weeks), hypoglycaemia and intracranial haemorrhage. The last may be related to trauma during delivery, although it may also occur postpartum in severe respiratory distress. The neonate is more likely to require resuscitation. Necrotising enterocolitis and patent ductus arteriosus are also more common in premature neonates. Longterm problems include cerebral palsy (in around 20% of babies born at 22-26 weeks, with 7% severe), neurodevelopmental delay and chronic lung disease. If maternal infection is suspected, neonatal screening is performed, since infection may also be present in the baby. Survival rates are approximately 40-45% for babies born at 24 weeks, and 80% for babies born at 26 weeks. It should be remembered that even with modern neonatal intensive care, the neonate has a greater risk of morbidity when born at 35-36 weeks than at 37-38 weeks.
Interpreting cardiotocographic (CTG) monitoring for preterm babies may be difficult, and fetal blood sampling is not performed before 34 weeks’ gestation, which may lead to an increase in expedited deliveries.
Management options
‘Rescue’ cervical cerclage may be offered to women between 16 and 28 weeks of pregnancy with a dilated cervix and exposed, unruptured fetal membranes if there are no signs of infection or contractions (see Chapter 7, Cervical suture).
Management of delivery includes finding the cause and ensuring that the timing of delivery is optimised. Steroids are given to the mother to aid maturation of the fetal lungs and are of benefit for up to seven days. Magnesium sulfate for neuroprotection is offered to all women between 24 and 30 weeks’ gestation who are in preterm labour, and should be considered in those between 30 and 34 weeks. Since steroids and magnesium require 24 hours to become optimally effective, delivery is usually delayed for this period if possible. Tocolytic drugs are commonly used in an attempt to prevent or stop labour, but their use is controversial, as the evidence suggests that although they may delay labour, they do not improve perinatal outcome. Antibiotics have been shown to reduce the incidence of preterm labour in women with PROM. Delivery is required in the presence of chorioamnionitis or fetal distress, although the precise mode of delivery is controversial. Since the preterm infant is more susceptible to intracranial haemorrhage, the need to prevent trauma during delivery often leads to caesarean section although the benefit of this is unproven.
Anaesthetic options are discussed more fully under the relevant related topics. In general, regional analgesia is often preferable in labour and is considered safe in the absence of systemic features of infection and if antibiotic cover has been provided, since it provides good conditions for a controlled delivery and can be readily extended for instrumental delivery. If caesarean section is required, regional anaesthesia may offer the parents their only chance to see and hear their baby free of tubes and other equipment if the chance of neonatal survival is poor. In addition, neurobehavioural and physiological outcome is better in premature neonates when regional anaesthesia is used than with general anaesthesia. It is important to appreciate the dangers of concurrent tocolytic therapy with any anaesthetic technique. The preterm fetus is especially vulnerable to the adverse effects of maternal hypotension.
Key points
• Between 5% and 10% of deliveries in the UK are preterm.
• Potential maternal problems are those of fever and sepsis, use of tocolytic drugs and the increased requirement for instrumental delivery and anaesthetic intervention.
• Fetal and neonatal problems are those of prematurity, infection and the increased need for neonatal resuscitation.
Further reading
National Institute for Health and Care Excellence. Preterm Labour and Birth. NICE Guideline NG 25. London: NICE, 2015. www.nice.org.uk/guidance/ng25 (accessed December 2018).
Simhan HN, Canavan TP. Preterm premature rupture of membranes: diagnosis, evaluation and management strategies. BJOG 2005; 112 (Suppl 1): 32-7.
Simhan HN, Caritis SN. Prevention of preterm delivery. N Engl J Med 2007; 357: 477-87.