While there are many known and unknown causes of preterm birth, women identified as being at risk may benefit from advice about risk and protective factors.
Preterm birth is defined as birth before 37 completed weeks of pregnancy . Sub-categories of preterm birth are based on weeks of gestational age: early preterm (<34 weeks), very preterm (28 to <32 weeks) and extremely preterm (<28 weeks). This section is concerned with spontaneous preterm birth as opposed to planned preterm birth.
23.1.1 Incidence of preterm birth
In Australia in 2014:
- overall, 8.6% of babies were born preterm, with most of these births occurring at gestational ages between 32 and 36 completed weeks
- the average gestational age for all preterm births was 33.3 weeks
- babies whose mothers smoked during pregnancy were more likely to be born preterm (13%) than those whose mothers did not smoke during pregnancy (8%).
- other characteristics associated with increased likelihood of preterm birth included:
- babies born in multiple births: 63% of twins and all (100%) of other multiples (triplets and higher) were preterm, compared with 7% of singleton babies
- babies born to mothers usually residing in more remote areas: 13% in very remote areas compared with 8% in major cities
- babies of younger (<20 years) and older (≥40 years) mothers: 11% and 12% were preterm, compared with 8% of babies with mothers aged 20–39 years.
Nationally in 2014, approximately 14% of babies of Indigenous mothers were born preterm, compared with 8% of babies of non-Indigenous mothers aOR 1.19, 95%CI 0.77 to 1.87, Indigenous mothers aOR 0.97 95%CI 0.52 to 1.80) .; similar rates were found in an earlier West Australian study (14.8 and 7.6%) . However, a study in a Melbourne hospital found no significant difference in risk of preterm birth between Indigenous and non-Indigenous babies and mothers (Indigenous babies
23.1.2 Risks associated with preterm birth
Preterm birth is associated with perinatal mortality, long-term neurological disability (including cerebral palsy), admission to neonatal intensive care, severe morbidity in the first weeks of life, prolonged hospital stay after birth, readmission to hospital in the first year of life and increased risk of chronic lung disease . Preterm birth can have a serious emotional impact on the family. In Australia in 2014 :
- preterm babies were more likely to be admitted to a special care nursery or neonatal intensive care unit (72%) than babies born at term (10%) or post-term (13%)
- spontaneous preterm birth accounted for 14% of all perinatal deaths and one third (33%) of perinatal deaths of babies of Indigenous mothers.
23.2 Identifying women at increased risk of giving birth preterm
A 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, addressing modifiable risk factors may reduce risk of preterm birth. It should also be noted that many women who experience preterm birth have no risk factors.
23.2.1 Significant risk factors
There is a significant association between preterm birth and:
- social disadvantage (OR 1.27, 95%CI: 1.16 to 1.39) and lower levels of maternal education (RR 1.48; 95%CI 1.29 to 1.69)
- previous preterm birth (absolute recurrence rate among women with a singleton pregnancy and previous preterm singleton birth 20%, 95% CI 19.9–20.6)
- pre-existing (p=0.002) or gestational diabetes
- current urogenital infections: eg chlamydia [OR 1.60; 90%CI 1.01 to 2.5] , bacterial vaginosis [OR 1.85; 95%CI 1.62 to 2.11]
- alcohol consumption (OR 1.34; 95%CI 1.28 to 1.41) , in a dose-response fashion
- smoking at the first antenatal visit (aOR 1.42, 95%CI 1.27 to 1.59) and active smoking during pregnancy (aOR 1.53, 95%CI 1.05 to 2.21) , with risk further increased among women smoking more than 10 cigarettes a day compared to those smoking 1–9 cigarettes per day (aOR 1.69 vs 1.54) .
23.2.2 Other factors
Systematic reviews of RCTs found:
- women who were overweight and obese who participated in aerobic exercise for 30–60 minutes three to seven times per week had a lower risk of preterm birth <37weeks (RR 0.62, 95% CI 0.41 to 0.95) compared to controls
- no clear difference in risk of preterm birth <37 weeks with treatment of periodontal disease (RR 0.87; 95%CI 0.70 to 1.10; low quality evidence) .
Systematic reviews of observational studies show the following associations with preterm birth:
- country of origin/ethnicity: odds of very preterm birth among East African immigrants were higher than among Australian-born women (aOR 1.55, 95%CI 1.27 to 1.90) and higher among African American women than among Caucasian women (pooled OR 2.0; 95%CI 1.8 to 2.2), with no significant association for Asian or Hispanic ethnicity
- weight: risk was increased among women who were obese and gained more than the IOM recommendations (aOR 1.54; 95% CI 1.09 to 2.16)
- emotional health and wellbeing: increased risk was associated with low social support compared to high social support (OR 1.22, 95%CI 0.84 to 1.76); stress (OR 1.52, 95%CI 1.18, to 1.97) ; untreated depression (OR 1.56; 95%CI 1.25 to 1.94) and anxiety (RR 1.50, 95%CI 1.33 to 1.70) , (OR 1.70, 95%CI 1.33 to 2.18) but not with maternal personality traits
- exposure to antidepressants: risk was increased among women exposed to antidepressants during pregnancy compared to women with depression but without antidepressant exposure (OR 1.17, 95%CI 1.10 to 1.25) , (RR 2.85, 95%CI 2.00 to 4.07) ; and risk was significantly increased with exposure in the third trimester (aOR 1.96, 95%CI 1.62 to 2.38) but not in the first trimester (aOR 1.16, 95%CI 0.92 to 1.45)
- environmental factors: increased risk was associated with high environmental temperature OR 1.20, 95%CI 1.07 to 1.34) or at home (OR 1.16, 95%CI 1.04 to 1.30) ; risk associated with exposure to fine particulate matter was unclear due to significant heterogeneity between studies , especially heat stress ; exposure to passive smoke in any place (
- pre-existing conditions: risk of preterm birth was increased among women with hepatitis C (OR 1.62, 95%CI 1.48 to 1.76, P < 0.001) , human papilloma virus (OR 2.12, 95%CI 1.51 to 2.98, P<0.001) , hypothyroidism (OR 1.19, 95%CI 1.12 to 1.26; P < 0.00001) and hyperthyroidism (OR, 1.24, 95%, CI 1.17- 1.31; P < .00001) but not hepatitis B (OR 1.12, 95%CI 0.94 to 1.33) .
- lifestyle factors: incidence of preterm birth (4.5% vs 4.4%; RR 1.01, 95%CI 0.68 to 1.50) was similar among women in the normal BMI category undertaking aerobic exercise during pregnancy and controls ; risk was increased among women with serum vitamin D levels lower than 50 nmol/L (OR 1.29, 95%CI 1.16 to 1.45) ; and there was no clear or statistically significant relationship between preterm birth and shift work , multivitamin use or influenza vaccination during pregnancy
- history of gynaecological procedures: risk was increased among women with a history of dilatation and curettage (D&C) (OR 1.29, 95% CI 1.17 to 1.42) or multiple D&Cs (OR 1.74, 95%CI 1.10 to 2.76) ; surgically induced termination of pregnancy (OR 1.52, 95%CI 1.08 to 2.16); surgically managed miscarriage (OR 1.19, 95%CI 1.03 to 1.37) ; loop electrosurgical excision procedure compared to women with no history of cervical dysplasia (pooled RR 1.61, 95%CI 1.35 to 1.92) but not when compared to women with a history of cervical dysplasia but no cervical excision (pooled RR 1.08, 95%CI 0.88 to 1.33) ; and treatment for cervical intraepithelial neoplasia before (OR 1.4, 95%CI 0.85 to 2.3) or during pregnancy (OR 6.5, 95%CI 1.1 to 37) .
When women are identified as being at risk of giving birth preterm based on the presence of risk factors, provide advice about modifiable risk factors.
Approved by NHMRC in October 2017; expires October 2022
23.3 Prediction and prevention
23.3.1 Cervical length measurement
Systematic reviews of randomised controlled trials found:
- among women with threatened preterm labour, those whose cervical length had been measured had a significantly lower rate of preterm birth <37 weeks (22.1 vs 34.5%; RR 0.64; 95%CI 0.44 to 0.94; 3 studies); management of women with a cervical length lower than the study threshold differed between studies (further observation in one study and administering tocolytics and antenatal corticosteroids in the other studies)
- no difference in incidence of maternal and neonatal infection among women with preterm premature rupture of the membranes who did or did not undergo transvaginal ultrasound of cervical length measurement .
Systematic reviews of observational studies were heterogeneous in terms of population and cut-off thresholds used but suggest that preterm birth is better predicted at 14 to 20 weeks rather than later, using a shorter cervical length as the cut-off threshold.
The evidence on cervical length measurement is emerging and will be reviewed as part of the next update of these Guidelines (anticipated for release in late 2019).
23.3.2 Holistic preventive strategies
Systematic reviews that evaluated holistic models of care and their effect on preterm birth found:
- a significant effect in reducing risk of preterm birth among women receiving midwifery-led care compared to other models of care for childbearing women and their infants (average RR 0.76, 95%CI 0.64 to 0.91; n=13,238; 8 studies; high quality)
- no significant difference among:
- women receiving group antenatal care compared to those receiving standard care (RR 0.87, 95%CI 0.70 to 1.09; 11 studies) and (RR 0.75, 95%CI 0.57 to 1.00; 3 3 studies; n=1,888, moderate quality)
- women randomised to specialist preterm birth programs compared to those receiving standard care (RR 0.92, 95%CI 0.76 to 1.12; 15 RCTs)
- low risk women receiving a reduced number of antenatal visits (RR 1.02, 95%CI 0.94 to 1.11; 7 studies, n=53,661, moderate quality)
- women receiving additional social support compared to those receiving standard care (RR 0.92, 95%CI 0.83 to 1.01; 11 RCTs; n=10,429) , including adolescent women (RR 0.67; 95%CI 0.42 to 1.05; 4 studies; n=684)
- women receiving telephone support during pregnancy compared to women receiving routine care or other support (RR 0.91, 95%CI 0.77 to 1.08, 4 RCTs; n=3,992)
- women in preterm labour using relaxation techniques compared to those not using relaxation techniques (RR 0.95; 95%CI 0.57 to 1.59; 11 RCTs; n=833)
- successful approaches to increasing access to antenatal care and reducing preterm birth among Aboriginal and Torres Strait Islander women include community-based collaborative antenatal care and community-based support and partnership between Aboriginal grandmothers, Aboriginal Health Officers, midwives and existing antenatal care services .
23.4 Discussing risk of giving birth preterm
When risk of preterm birth is increased, modifiable risk factors should be addressed. Based on the evidence discussed in Section 23.2, discussion with women at risk of preterm birth can include the benefits of:
- having adequate social and emotional support
- quitting tobacco smoking and avoiding exposure to passive smoke
- not drinking alcohol during pregnancy
- having tests for urogenital infections
- participating in regular exercise, particularly if they are overweight or obese.
Women can also be advised that risk is not reduced by supplementing with Vitamins C or Eor probiotics .
A Cochrane review found no evidence to support or refute bed rest for prevention of preterm birth. A subsequent cohort study found that, among women at high risk of preterm birth, activity restriction was associated with increased risk of preterm birth .
23.5 Practice summary: risk of preterm birth
A woman has identified risk factors for giving birth preterm
- Aboriginal and Torres Strait Islander Health Practitioner
- Aboriginal and Torres Strait Islander Health Worker
- multicultural health worker
Discuss lifestyle factors associated with preterm birth
- Explain that smoking during pregnancy makes it more likely that the baby will be born preterm and also causes other serious risks to the pregnancy.
- Explain that not drinking alcohol during pregnancy is the safest option.
- Offer testing for urogenital infection if the woman has risk factors for preterm birth. If results are positive, consider counselling, contact tracing, partner testing and treatment, and repeat testing.
Discuss protective factors
- Explain that moderate physical activity during pregnancy has a range of health benefits, particularly for women who are overweight or obese.
Take a holistic approach
- Provide information on relevant community supports (eg smoking cessation programs, drug and alcohol services, physical activity groups).
- Consider whether a woman may be at increased risk if she has recently arrived from a country with a high prevalence of preterm birth.
- Provide social and emotional support and access to continuity of carer, where possible
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