Preterm labour

Published on 09/03/2015 by admin

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Chapter 37 Preterm labour

Justin Nasser

Definition. Preterm or premature labour is the onset of regular painful uterine contractions accompanied by effacement and dilatation of the cervix after 20 weeks and before 37 completed weeks of pregnancy.

Incidence: Preterm labour occurs in 5%–10% of all deliveries.

Common complications in premature infants include respiratory distress syndrome, intraventricular haemorrhage, bronchopulmonary dysplasia, patent ductus arteriosus, necrotising enterocolitis, sepsis, apnoea and retinopathy of prematurity.

The frequency of major morbidity rises as gestational age decreases.

Ongoing advances in neonatal medicine have resulted in dramatically improved outcomes for preterm infants; however, there remains significant risk of long-term morbidity, such as cerebral palsy, developmental delay, visual and hearing impairment, and chronic lung disease.

Significance

• Prematurity is the cause of 75% of perinatal deaths and a major determinant of short-term and long-term morbidity in infants and children.
• Two-thirds of the perinatal deaths occur in the 30%–40% of preterm infants who are delivered prior to 32 weeks gestation.

Aetiology

Preterm labour may be classified as spontaneous or indicated.

Spontaneous preterm labour

• Spontaneous preterm labour occurs in the absence of overt maternal or fetal conditions necessitating delivery. It commonly occurs in the absence of an obvious cause, or may follow preterm premature rupture of the membranes or related diagnoses such as incompetent cervix.
• Risk factors associated with spontaneous preterm labour include a history of previous preterm birth, multiple pregnancy, polyhydramnios, urogenital tract infection, previous cervical surgery, uterine anomalies, periodontal disease, bleeding in the second trimester, extremes of age, smoking, low prepregnancy weight and pregnancies achieved through assisted reproductive technologies.
• Most women who deliver preterm have no apparent risk factors.

Indicated preterm labour

• Indicated preterm labour occurs when conditions exist that create undue risk to the mother, the fetus, or both, should the pregnancy continue, and a clinical decision is made to expedite delivery. In these situations, the labour may be induced or the delivery achieved by caesarean section.
• The most common diagnoses that precede an indicated preterm birth are pre-eclampsia, fetal distress, intrauterine growth restriction, placental abruption and fetal demise.

Prevention of preterm labour

Many interventions aimed at reducing the incidence of preterm labour have been implemented and assessed, including:

• supplemental progesterone
• cervical cerclage
• detection and treatment of asymptomatic bacteriuria
• detection and treatment of bacterial vaginosis
• smoking cessation
• nutritional supplementation

None of these have shown consistent benefit. In fact, the incidence of preterm labour appears to be rising. The lack of success in preventing preterm labour can be attributed to:

• the current incomplete understanding of the physiology of normal parturition
• the current incomplete understanding of the pathogenesis of preterm labour
• the poor sensitivity and positive predictive value of currently available screening tests

Because of the difficulties of predicting and preventing preterm labour, the main goal of management is in early diagnosis of those at true risk of delivery, and implementation of therapies aimed at optimising perinatal outcomes.

Diagnosis of preterm labour

The clinical diagnosis of preterm labour is often unreliable, with up to 50% of women with signs and symptoms suggestive of preterm labour not progressing to preterm delivery.

A number of ancillary tests have been developed in an attempt to identify both asymptomatic and symptomatic women who are truly likely to deliver prematurely. The most clinically useful of these tests are:

• cervical length and morphology assessment by transvaginal ultrasound
• fetal fibronectin (fFN) detection in cervicovaginal secretions

Cervical length and morphology

• Well-defined, reproducible changes in the appearance of the cervix by transvaginal ultrasound occur as labour progresses.
• The cervix undergoes progressive shortening and widening along the endocervical canal commencing at the internal os (‘funnelling’).
• The initial changes are almost always asymptomatic and not identified by digital vaginal examination.
• A cervical length of <2.5 cm at 16–24 weeks gestation is a strong predictor of preterm birth when used as a screening test in both low-risk and high-risk women. The shorter the cervical length and the earlier in pregnancy that the shortening occurs, the greater the likelihood of preterm birth.
• In symptomatic women, a cervical length of >3 cm is likely to exclude the diagnosis of preterm labour.

Fetal fibronectin (fFN)

• fFN is a glycoprotein ‘glue’ that binds chorion to decidua.
• Disruption of the maternal fetal interface causes release of fFN into the cervicovaginal secretions.
• In a normal pregnancy, fFN should be almost undetectable in vaginal secretions from 22–35 weeks.
• The presence of fFN in the cervicovaginal secretions is a predictor of preterm birth; however, its clinical utility is in its negative predictive value, as <1% of women with a negative test will deliver within 1 week.

The use of both fFN and cervical sonography may increase the utility of these tests in assessing at-risk women.

Management of preterm labour

Because of the lack of success in predicting and preventing preterm labour, the aim of management is largely to reduce the likelihood and impact of prematurity-related sequelae. Three interventions have been shown to reduce perinatal morbidity and mortality in women who deliver preterm:

• in utero transfer to a facility with appropriate neonatal facilities
• administration of corticosteroids to the mother to facilitate fetal lung maturation and reduce complications of prematurity
• administration of antibiotics to prevent neonatal group B streptococcus infection

In the setting of preterm labour, tocolytic therapy aims to delay delivery in order to facilitate corticosteroid administration and in utero transfer to an appropriate facility for delivery.

There are a number of therapeutic agents that can suppress uterine muscle activity, but all have potential adverse side effects. Current evidence suggests that:

• Calcium channel blockers (e.g. nifedipine) or an oxytocin antagonist (atosiban) can delay delivery for 2–7 days with minimal side effects.
• Beta-agonists (e.g. ritodrine, terbutaline, salbutamol) are effective at delaying delivery for 48 hours, but are associated with greater side effects.
• Magnesium sulfate is an ineffective tocolytic.
• The benefit of cyclo-oxygenase inhibitors (e.g. indomethacin) is uncertain.

Much of the improvement in perinatal morbidity and mortality associated with prematurity is due to the advances in neonatal management.

Further reading

Grimes-Dennis J., Berghella V. Cervical length and prediction of preterm delivery. Current Opinion in Obstetrics and Gynecology. 2007;19:191-195.

Goldenberg R.L., Culhane J.F., Iams J.D., Romero R. Preterm birth 1. Epidemiology and causes of preterm birth. Lancet. 2008;371:75-84.

Iams J. Prediction and early detection of preterm labour. Obstetrics and Gynecology. 2003;101:402-412.

Iams J.D., Romero R., Culhane J.F., Goldenberg R.L. Preterm birth 2. Primary, secondary, and tertiary interventions to reduce the morbidity and mortality of preterm birth. Lancet. 2008;371:164-175.

Vidaeff A.C., Ramin S.M. Management strategies for the prevention of preterm birth. Part I: update on progesterone supplementation. Current Opinion in Obstetrics and Gynecology. 2009;21:480-484.

Vidaeff A.C., Ramin S.M. Management strategies for the prevention of preterm birth. Part II: update on cervical cerclage. Current Opinion in Obstetrics and Gynecology. 2009;21:485-490.

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