37

Prematurity

Introduction

Prematurity is defined as delivery between 24 and 37 weeks’ gestation and occurs in 6–10% of births. There is often no apparent predisposing cause for pre-term labour (idiopathic) but it is recognized to be more common with multiple pregnancy, antepartum haemorrhage, fetal growth restriction, cervical incompetence, amnionitis, congenital uterine anomaly, polyhydramnios and systemic maternal infection. Almost one-third of pre-term births in the UK are iatrogenic following deliberate medical intervention when the risk of continuing the pregnancy for either the mother or the fetus outweighs the risks of prematurity.

To be born prematurely is a potentially serious hazard. Fetal morbidity and mortality rates are inversely proportional to the maturity of organ systems, especially the lungs, brain and gastrointestinal tract. Prematurity especially before 33 weeks’ gestation is the leading cause of perinatal morbidity and mortality. It is exceptional to survive if delivered before 24 weeks’ gestation, especially without significant disability. In the most recent UK perinatal mortality report from the Centre for Maternal and Child Enquiries (CMACE 2011), 67% of neonatal deaths in 2009 occurred under 37 weeks’ gestation. Of those infants who survive, 10% will suffer some form of long-term disability requiring additional needs, and a greater number may suffer from lesser developmental or behavioural problems; these proportions are higher with earlier gestational age.

Research into mechanisms involved in pre-term delivery and its prevention has been relatively unsuccessful to date; as a result, prematurity is currently one of the most challenging problems facing both obstetricians and paediatricians.

Definitions

Pre-term – a gestation of less than 37 completed weeks

Very pre-term – a gestation of less than 32 completed weeks

Pre-term labour – regular uterine contractions accompanied by effacement and dilatation of the cervix after 20 weeks and before 37 completed weeks

Pre-term pre-labour rupture of the membranes (PPROM) – rupture of the fetal membranes before 37 completed weeks and before the onset of labour

Low birth weight (LBW) – birth weight of less than 2501 g. It is important to note that low-birth-weight infants may be pre-term or growth restricted or both (p. 35)

Very low birth weight (VLBW) – birth weight of less than 1501 g

Extremely low birth weight (ELBW) – birth weight of less than 1000 g

Perinatal mortality rates – see Chapter 48.

Aetiology and predisposing factors

The incidence of pre-term birth is 6–10% (Table 37.1) and approximately 1.5% will deliver before 32 weeks. Although only 0.5% deliver before 28 weeks, this latter group accounts for two-thirds of the neonatal deaths.

The aetiological factors that trigger spontaneous pre-term labour are largely unknown, but may be mediated through cytokines and prostaglandins. In some instances, it is related to increased uterine size or to other hormonal factors. Infection has been implicated in pre-term delivery and it may be that bacterial toxins initiate an inflammatory process in the chorioamniotic membranes, which in turn release prostaglandins. Bacteria may damage membranes by direct protease action, or by stimulating production of immune mediators like 5-hydroxytryptamine, which stimulate smooth muscle cells. None of these postulated mechanisms satisfactorily explains every case of pre-term labour and the aetiology is therefore considered to be multifactorial.

Identifying women at increased risk of pre-term birth

A number of techniques or strategies have been proposed to identify pregnancies at increased risk of pre-term birth. They include clinical risk scoring, bacteriological assessment of the vagina, cervical assessment and the measurement of fetal fibronectin (Ffn).

The ‘risk scoring’ is based on the recognized risk factors available from the maternal obstetric, gynaecological and medical histories together with smoking status, body weight and socioeconomic status. The strongest of these is a history of previous pre-term birth but this is clearly not applicable to first-time mothers. Other recognized associations are presented in Box 37.1. Unfortunately, the performance of these scoring systems has been poor, with sensitivities quoted as less than 40%, but the system at least serves to highlight the potentially avoidable factors such as urinary tract infections, vaginal infections, smoking, drugs and chaotic lifestyle.

Screening for and treating vaginal infection has been considered as a mechanism to identify and treat those at high risk of pre-term labour. Conflicting results have been demonstrated. Bacterial vaginosis, which is present in 10–20% of pregnant women, is associated with a doubling of the risk of pre-term delivery if identified in the third trimester, but a five-fold increased risk if identified in the first or early second trimester. Unfortunately, clinical trials have not demonstrated that treating bacterial vaginosis reduces the risk of pre-term labour. It is reasonable to treat bacterial vaginosis identified in early pregnancy, in those considered to have other risk factors for pre-term labour.

More recently, interest has focused on transvaginal ultrasound in measurement of the cervix. A normal cervical length is between 34 and 40 mm and there should be no funnelling at the internal os (Fig. 37.1)

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