Clinical Practice Guidelines Antenatal Care — Module II
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- Incidence of preterm birth
- Risks associated with preterm birth
- Identifying women at increased risk of giving birth preterm
- Significant risk factors
- Access to antenatal care
- Discussing risk of giving birth preterm
- Practice summary: risk of preterm birth
6.10Risk of preterm birthWhile the causes of preterm birth are multifactorial, women identified as at risk may benefit from advice about risk and protective factors. BackgroundPreterm birth is defined as birth before 37 completed weeks of pregnancy (WHO 2012). Sub-categories of preterm birth are based on weeks of gestational age (WHO 2012): extremely preterm (<28 weeks); very preterm (28 to <32 weeks); and moderate to late preterm (32 to <37 weeks). Incidence of preterm birthGlobally, 15 million babies are born preterm each year and 1.1 million die as a result of complications (WHO 2012). The majority (60%) of preterm births occur in Africa and South Asia (Beck et al 2010). The risk of preterm birth is higher among Indigenous populations in Australia, Canada and the United States (Heaman et al 2005; Shah et al 2011). Preterm birth rates are rising globally (Langhoff-Roos et al 2006; Keirse et al 2009; Schaaf et al 2011; WHO 2012), including in Australia (Roberts et al 2003; Tracy et al 2007). In Australia in 2010, 8.3% of babies were born preterm. Most of these births occurred at a gestational age of 32–36 completed weeks (Li et al 2012). The Northern Territory had the highest proportion of preterm births, at 10.6% of all births, and New South Wales had the lowest, at 7.4% of all births. Preterm birth is a significant cause of morbidity and mortality among infants of Aboriginal and Torres Strait Islander mothers. In 2010, 13.5% of babies of Aboriginal and Torres Strait Islander mothers were born preterm, compared to 8.0% of babies of non-Indigenous mothers (Li et al 2012).
Summary of the evidenceA range of risk and protective factors influence the likelihood of preterm birth. While many risk factors are not modifiable during a woman’s current pregnancy, it may be possible to modify their effects. Women at high risk of preterm birth should be referred for specialist assessment and management. Significant risk factorsThere is a significant association between preterm birth and: social disadvantage and inequality (DeFranco et al 2008; Gray et al 2008); previous preterm birth (Heaman et al 2005; McPheeters et al 2005; Kiran et al 2010; Heaman et al 2013); urogenital infections: chlamydia (see Module I, Section 8.5), bacterial vaginosis (Module I, Section 8.8), gonorrhoea (see Section 8.11) and trichomoniasis (see Section 9.1); alcohol consumption, in a dose-response fashion (Sokol et al 2007; Aliyu et al 2010; Avalos et al 2011; Patra et al 2011); active smoking during pregnancy, with risk further increased among heavy smokers (Kyrklund-Blomberg et al 2005; Fantuzzi et al 2007; Wills & Coory 2008; Freak-Poli et al 2009; Bickerstaff et al 2012); pre-existing diabetes (HAPO Study Cooperative Research Group 2008; Köck et al 2010); and depression (Dayan et al 2006; Grote et al 2010; Fransson et al 2011), although some studies found an association only in women on medication for depression (Suri et al 2007; Gavin et al 2009), which may reflect either an effect of medication or severity of depression. Other factorsAge: Women younger than 20 years had a higher risk of preterm birth than women aged 20–35 years (Gupta et al 2008) and higher rates of extreme prematurity (<28 weeks) than women aged 20–39 years (Shrim et al 2011). Women aged more than 35 years also had a greater risk of preterm birth than women aged 25–29 years (McIntyre et al 2009; Schure et al 2012). Weight: Low (Siega-Riz et al 1996; Panaretto et al 2006; Khashan & Kenny 2009) or high (Viswanathan et al 2008; McDonald et al 2010) pre-pregnancy BMI were associated with preterm birth. Pregnancy history: Previous termination of pregnancy (Moreau et al 2005; Freak-Poli et al 2009; Gagnon et al 2009; Heaman et al 2013) and short inter-pregnancy interval (less than 6 months between birth and the conception of the next baby) (Rodrigues & Barros 2008; Wendt et al 2012) were associated with preterm birth. Periodontal disease treatment: Some studies found a decreased risk with treatment (Scannapieco et al 2003; Polyzos et al 2009; Pimental Lopes de Oliveira 2010), some found no decrease in risk (Michalowicz et al 2006; Newnham et al 2009; Offenbacher et al 2009; Macones et al 2010; Polyzos et al 2010; Chambrone et al 2011; George et al 2011) and others were inconclusive (Agueda et al 2008; Africa 2011). Pregnancy-related anxiety: Some studies found a significant association (Dole 2003; Orr et al 2007), while others found no association (Dayan et al 2006). Passive smoking: Risk increased with the number of smokers in the house (Fantuzzi et al 2007) and there is emerging evidence of a reduction in preterm births following introduction of public smoking bans (Cox et al 2013). Hypertension: There is low-level evidence that hypertension with other risk factors predisposes women to preterm birth (Vreeburg et al 2004). Illicit drug use: Use of amphetamines, opiates and marijuana was associated with preterm birth (Ludlow et al 2004; Hayatbakhsh et al 2012). Access to antenatal careInadequate antenatal care is strongly associated with preterm birth in adolescents (OR: 7.4; 95% CI: 5.7–9.7 for no antenatal care compared with 75–100% of recommended visits) (Debiec et al 2010). A recent literature review found that group antenatal care (in which women receive antenatal care and education in a group environment) increased attendance among young women and reduced incidence of preterm birth (Allen et al 2012). A community-based collaborative approach to antenatal care for Aboriginal and Torres Strait Islander women in Townsville increased access to antenatal care and was associated with a significant reduction in preterm births (Panaretto et al 2005; Panaretto et al 2007). An Aboriginal medical service midwifery program in the ACT with a similar approach showed a slight reduction in preterm births (Wong et al 2011). A systematic review found insufficient evidence to conclude that alternative models of organising or delivering antenatal care reduce preterm birth in socially disadvantaged or vulnerable populations compared with standard models of antenatal care (Hollowell et al 2009).
Protective factorsA literature review (Domingues et al 2009) and cohort studies (Hegaard et al 2008; Juhl et al 2008; Owe et al 2012) identified an association between reduced risk of preterm birth and involvement in leisure-time physical activity during pregnancy (compared to sedentary behaviours). Emerging evidence on marine n-3 fatty acids (Salvig & Lamont 2011) and fish consumption (Haugen et al 2008) is inconclusive. Recommendation 16 Grade B Advise women at risk of giving birth preterm about risk and protective factors. Discussing risk of giving birth pretermWhen risk of preterm birth is increased, modifiable risk factors should be addressed (Freak-Poli et al 2009; Kiran et al 2010; Carter et al 2011). Based on the evidence discussed in Section , discussion with women at risk of preterm labour can include the benefits of: quitting tobacco smoking and avoiding passive smoking; not drinking alcohol during pregnancy; having tests for urogenital infections; and being involved in leisure-time physical activity. Women can also be advised that risk is not reduced by supplementing with Vitamins C or E (Rumbold & Crowther 2005; Hauth et al 2010) or probiotics (Othman et al 2007; Hauth et al 2010). Practice summary: risk of preterm birth
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