8
Preterm delivery

Up to 12% of deliveries in developed countries are preterm. In the US, 11.5% of births were preterm in 2012. In the UK, 7.2% of births were preterm, but only 5.9% in Sweden and Japan (Fig. 8.1). The increase in the proportion of infants born preterm in the US since 1980 is shown in Fig 2.3.

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Fig. 8.1 Preterm birth rate in different countries in 2010, showing the high rate in the US and moderately high rate in the UK.

(Adapted from Chang HH et al. Preventing preterm births: analysis of trends and potential reductions with interventions in 39 countries with very high human development index. Lancet 2013;381:223–34.)

Causes

Neonates may be born preterm following:

  • spontaneous labor with intact membranes − 40 to 45%
  • preterm premature rupture of the membranes (PPROM) – 25 to 30%
  • labor induction or cesarean delivery for maternal or fetal indications − 30 to 35%.

The main causes are shown Fig. 8.2.

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Fig. 8.2 Causes of prematurity.

(IUGR, intrauterine growth restriction; PPROM, preterm prolonged rupture of the membranes.)

Epidemiological risk factors

There are many risk factors which are poorly understood but generally predispose the mother to infection or inflammation.

These include:

  • Previous preterm delivery – twofold increased risk, increasing for each additional preterm delivery.
  • Short inter-pregnancy interval of <6 months − more than doubles the risk.
  • Maternal age − increased risk if <20 or >35 years old.
  • Maternal nutrition – low BMI (body mass index) increases risk of spontaneous preterm birth. Obese mothers are more likely to have preterm births for medical indications, particularly pre-eclampsia and diabetes mellitus.
  • Ethnicity − whereas the preterm rate in the US is 10−11% in White or non-Hispanic mothers, it is 16 −18% in Black mothers. Women from south Asia have high rates of low-birthweight infants rather than preterm delivery.
  • Multiple births − result in 15−20% of preterm births. Early delivery is recommended for monochorionic twins by 36 weeks in the UK, 37 weeks in US.
  • High levels of maternal psychological or social stress − increased risk − generally less than twofold.
  • Smoking − increased risk–less than twofold.
  • Substance misuse.
  • Socio-economic deprivation − inter-relationship poorly defined.
  • Maternal health − infections, either localized, i.e. ascending infection, or generalized, e.g. malaria.

Prevention

Strategies to prevent preterm labor include:

  • Progesterone − given prophylactically from 24 weeks, reduces preterm birth and perinatal mortality in those at high risk of preterm labor, e.g. previous preterm birth or short cervix identified on ultrasound, but not multiple births.
  • Cervical ‘cerclage’ − purse-string suture to maintain closure of the maternal cervix. Benefit uncertain, but often offered if multiple preterm births, mid-trimester fetal losses or cervix is shortening. Non-surgical ‘cervical pessary’ is being investigated.
  • Genital infections, e.g. bacterial vaginosis, where overgrowth of anaerobic vaginal organisms displaces normal lactobacillus species. Remains controversial, may be treated.
  • Cessation of maternal smoking.
  • Reduction in multiple births by limiting embryo transfer in IVF treatment.
  • Reduction in elective preterm deliveries − (see below).

However, the potential impact of these interventions to reduce the proportion of infants born preterm is relatively small.

Management

Antenatal corticosteroids

Maternal corticosteroids administered before preterm birth reduce rates of respiratory distress syndrome by 44%, intraventricular hemorrhage by 46% and neonatal death by 31%. Also associated with a reduction in necrotizing enterocolitis, respiratory support, intensive care admissions and systemic infections in the first 48 hours of life (see Fig. 67.2).

A single course is administered to mothers at risk of preterm birth up to 35 weeks of gestation. In the UK it is also offered to women having an elective cesarean section prior to 39 weeks’ gestation to reduce the risk of respiratory morbidity.

Preterm premature rupture of the membranes (PPROM)

Affects 2−3% of pregnancies, but is associated with 25−30% of preterm deliveries. Increases neonatal morbidity and mortality due to prematurity, infection and pulmonary hypoplasia. Associated with ascending maternal infection from the lower genital tract; about one-third have positive amniotic fluid cultures. Antibiotics reduce chorioamnionitis and neonatal infection. The decision to deliver or manage expectantly requires balancing of risk of intrauterine infection compared with neonatal risks from prematurity. If < = 34 weeks, corticosteroids are usually given. Beyond 34 weeks’ delivery is usually indicated.

Tocolysis

Used to suppress uterine contractions. No clear evidence that any improve outcomes, but widely used to try to suppress contractions to enable completion of course of antenatal corticosteroids or allow maternal transfer to a perinatal center.

Magnesium sulfate

Offered to mothers shortly before preterm delivery at 24−32 weeks’ gestation to reduce the risk of cerebral palsy. Several trials have shown a 30−40% reduction in cerebral palsy rates, (number needed to treat 63 to prevent one case of cerebral palsy).

Delivery

The aim is to prolong pregnancy for as long as possible while ensuring the safety of the mother and fetus.

  1. Extreme preterm delivery (<28 weeks)

    Deciding about the timing of a preterm delivery is most difficult at the limit of viability (22−26 weeks) and should involve the obstetrician, neonatologist and parents. Decision-making is helped by a detailed assessment of fetal well-being including assessment of amniotic fluid volume, fetal heart rate monitoring, Doppler studies, fetal growth, gestation and predicted birthweight (with estimates of their accuracy). This should also be informed by knowledge of outcomes at these early gestational ages. National and international data are available, but will need to be modified according to the circumstances. Delivery of high-risk infants should occur at a perinatal center to avoid subsequent transfer and separation of the infant and mother.

  2. Delivery at 34–38 weeks

    Although much of the attention of neonatologists (and a significant proportion of this book) is focused on the extremely preterm infant, attention has recently turned to outcomes of infants delivered at 34–38 weeks. These infants have an increase in respiratory morbidity and increased length of stay in hospital compared with full term infants born at 39–<41 weeks. Although the neurodisability rate is highest in extremely preterm infants, rates are higher in these infants than those born at full term (Figs 8.3 and 38.2). There has been a marked reduction in the number of infants delivered before 39 weeks’ gestation in the US following new guidelines.

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Fig. 8.3 Prevalence of special educational needs (SEN) by gestational age at birth, showing increased proportion even at 34–39 weeks compared with full-term births. Data based on 407 503 school-aged children in Scotland in 2005.

(Source: MacKay D.F. et al. PLoS Medicine 2010; 7(6): e1000289.)