Introduction

Labour is defined as preterm when it occurs before the end of the 37th week of pregnancy. Babies are described in terms of either birthweight or gestational age. Infants delivered less than 37 completed weeks from the first day of the last menstrual period are referred to as preterm, irrespective of weight, while infants weighing less than 2500 g are classified as of low birthweight. Babies may be both preterm and of low birthweight.

Recent advances in neonatal care have resulted in the survival of very small and immature infants and these classifications have been expanded: very low birthweight infants (VLBW) are defined as those weighing less than 1500 g at birth and extremely low birthweight infants (ELBW) as less than 1000 g at birth. Some authorities add a further category which describes infants weighing 750 g or less: these may be referred to as incredibly low birthweight (ILBW) (Amon 1999).

It is important to differentiate between the low birthweight preterm infant and the baby whose birthweight is low because of intrauterine growth restriction, as each group has different needs and problems after birth.

The incidence of preterm delivery as a proportion of all births is around 7% in the UK, translating into roughly 50,000 babies every year. Rates of preterm delivery increase with increasing gestational age, up to 37 weeks, with less than a quarter occurring before 32 weeks. Preterm birth is directly responsible for 75–90% of all neonatal deaths not due to lethal congenital malformations and is a major cause of both short-term and long-term neonatal morbidity (Amon 1999).

Aetiology

Preterm birth may occur as a result of any of the following situations:

Prediction and prevention of preterm labour

Several methods have been used to try to identify women at risk of preterm labour, though prediction is difficult and may not be effective in preventing preterm birth. Formal risk-scoring systems have been used, based on the factors described above. This method has relatively poor predictive value for spontaneous preterm labour, especially for primigravid women; a low score may induce a false sense of security (Amon 1999). However, it may be more useful where it is possible to identify factors relating to past obstetric history and current events.

Regular pelvic examination: The difficulty with digital examination of the cervix is the lack of consistency between examinations and examiner, though it may reveal signs of cervical changes that may herald the onset of labour. However, this procedure may of itself introduce infection and has a low predictive value for women at risk of preterm birth (Colombo 2002, Enkin et al 2000).

Ultrasonographic measurement of cervical length may be an accurate predictive tool in high-risk women. Funnelling of the internal cervical os may indicate impending preterm labour; however, there is no cervical length below which all women deliver prematurely and no cervical length above which none of the women deliver early. The only conclusion that can be made is an increased risk for early delivery with a shortened cervix (Colombo 2002).

Fetal fibronectin tests may predict the onset of preterm labour (Lockwood et al 1991). Fetal fibronectin is a component of the extracellular matrix, secreted by the anchoring trophoblastic villi. Although its presence in vaginal secretions is a normal finding in the first half of pregnancy, concentrations greater than 50 ng/mL after 22 weeks’ gestation are indicative of chorio-decidual disruption (Amon 1999).

If preterm labour is imminent, there is separation of maternal and fetal tissue at the chorio-decidual junction, leading to leakage of fibronectin. However, false positive results can occur, which means that some women may be subjected to unnecessary interventions. The test should be carried out every 2 weeks from 24 weeks’ gestation and cannot be used in the presence of vaginal bleeding or rupture of the membranes, as both blood and amniotic fluid contain fibronectin.

Vaginal pH monitoring may be useful, as a rise in pH often indicates the presence of infection, which may precipitate labour.

Fetal breathing movements cease before preterm labour commences, and although this may not occur in every case, it is a reasonably reliable indicator of imminent preterm delivery (Amon 1999, Castle & Turnbull 1983). However, the time and resources required to screen large populations of at-risk women may make this unattractive as a screening procedure.

Prevention of preterm birth is dependent upon preventing uterine activity and/or cervical dilatation. In the past, bedrest was advocated as a preventive measure, though now has been shown to be ineffective, and it may actually increase the risk for deep vein thrombosis (Enkin et al 2000, Kovacevich et al 2000). Antibiotic therapy has been used in the management of women at risk of preterm labour, though there is no powerful evidence to support the routine use of antibiotics where the membranes are intact. A large randomized study of metronidazole treatment for asymptomatic bacterial vaginosis also failed to demonstrate any reduction in the incidence of preterm labour (Carey et al 2000, Enkin et al 2000, King & Flenardy 2000).

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