preterm labour

Last edited 05/2022 and last reviewed 05/2023

Preterm labour is defined as occuring before 37 weeks of gestation. This is the point where obstetrics and neonatal paediatrics firmly overlap. For an obstetrician looking after a woman in premature labour it is essential to involve the paediatricians. Co-operation between the two teams should be well-established by the time of birth.

Important information which may influence management includes:

• circumstances that triggered labour
• gestational age
• prenatal drugs

Preterm birth, defined as birth occurring before 37+0 weeks’ gestation, is the single largest cause of morbidity and mortality in the UK (1)

• several pregnancy pathologies are associated with an increased risk of indicated or spontaneous preterm birth, some of these are pre-existent condition, for example, chronic hypertension, pre-pregnancy diabetes mellitus, systemic lupus erythematosus and maternal underweight or obesity
• short interpregnancy interval and a family history of preterm birth can also be indicators of higher risk of spontaneous preterm birth. However, some other pathologies are pregnancy dependent such as preeclampsia or gestational diabetes mellitus.

Preterm birth can also occur in low risk asymptomatic pregnancies as the symptom or outcome of many different aetiological processes such as infection, bleeding, uterine over-distention, cervical weakness (1).

Preterm birth itself is not the negative event that has to be prevented, however it is associated with various complications for the newborn, including increased risk of respiratory distress syndrome, intraventricular haemorrhage, retinopathy of prematurity and neonatal mortality, with risk of neurodevelopmental disability in the longer term

• risk of complications and mortality increases with decreasing gestational age at birth
• are currently around 55,000 preterm births each year in England and Wales, an annual incidence of around 7.9%.2 The DH Maternity Safety Ambition is to reduce the national preterm birth rate to 6% (2)

Around three-quarters of preterm births are spontaneous, following onset of preterm labour or preterm prelabour rupture of membranes (P-PROM) rather than as a result of medically-indicated ‘iatrogenic’ preterm deliver (1)

• mechanisms behind spontaneous preterm birth may be multifactorial, including infective or inflammatory processes, cervical dysfunction, nutritional, socioeconomic and environmental influences (3) Various risk factors are also known to be associated including: (1,4)
  • multiple pregnancy
  • history of preterm birth <34 weeks
  • history of mid-trimester loss (16 to 24 weeks)
  • history of P-PROM <34 weeks
  • uterine anomalies
  • cervical trauma/cervical surgical procedure

Certain clinical findings or biological markers have been found to be associated with a higher risk of preterm birth and are currently used for selective testing of symptomatic women or certain high-risk groups, notably cervical length measurement

• in the UK pregnant women who have history of preterm birth or mid-trimester loss and a short cervical length (<25mm) in the mid-trimester may be offered prophylactic vaginal progesterone or cervical cerclage (stich). Similarly cervical cerclage may be considered for women with short cervix and history of P-PROM or cervical trauma (1)
• cervicovaginal fetal fibronectin (fFN) is another biological marker from the placental/fetal membranes that is usually at low concentration (<50ng/ml) in mid-pregnancy, while only increasing at term
• concentration of fFN in the vaginal fluid may be measured in symptomatic women presenting in preterm labour to indicate the likelihood of birth and to triage admission or in utero transfer (1)
• common vaginal imbalance of bacterial vaginosis has also been associated with risk of preterm birth, and is usually treated with antibiotics if women are symptomatic or are found to have the condition incidentally in pregnancy. However, routine screening of asymptomatic women with no existing risk factors for preterm birth, by any test, is not currently performed in the UK
• bacterial vaginosis is another risk factor associated with preterm birth and related neonatal and maternal outcomes. It is the most common lower genital tract infection among women of reproductive age (studies suggesting prevalence of up to 1 in 4) (6)
• diagnosis is based on characteristic vaginal discharge, pH testing of vaginal discharge (>4.5) and swab for Gram staining. Bacterial vaginosis is currently treated with antibiotics if it is detected symptomatically in pregnancy

Infants born preterm (before 37 weeks' gestation) are at risk of respiratory distress syndrome (RDS) and need for respiratory support due to lung immaturity (7)

• one course of prenatal corticosteroids, administered to women at risk of preterm birth, reduces the risk of respiratory morbidity and improves survival of their infants
• a systematic review found that short term benefits for babies included less respiratory distress and fewer serious health problems in the first few weeks after birth with repeat dose(s) of prenatal corticosteroids for women still at risk of preterm birth >=7 days after an initial course

Reference:

• 1. NICE Preterm Labour and Birth. NICE guideline [NG25] [Internet]. London: National Institute for Health and Care Excellance; [updated August 2019]. Available from: https://www.nice.org.uk/guidance/ng25/chapter/Recommendations
• 2. DH. Safer Maternity Care - The National Maternity Safety Strategy - Progress and Next Steps. London: Department of Health and Social Care, 2017. Available from: https://www.gov.uk/government/publications/safer-maternity-care-progress-and-next-steps.
• 3. Honest H, Forbes C, Durée K, et al. Screening to prevent spontaneous preterm birth: systematic reviews of accuracy and effectiveness literature with economic modelling. Health Technol Assess. 2009;13(43):1-627.
• 4. Story L, Simpson NA, David AL, et al. Reducing the impact of preterm birth: Preterm birth commissioning in the United Kingdom. European journal of obstetrics & gynecology and reproductive biology: X. 2019.
• 5. Dos Santos F, Daru J, Rogozinska E, et al. Accuracy of fetal fibronectin for assessing preterm birth risk in asymptomatic pregnant women: a systematic review and meta-analysis. Acta Obstet Gynecol Scand.2018;97(6):657-67.
• 6. NICE. Antenatal care for uncomplicated pregnancies. Clinical Guideline [CG62] [Internet]. London: National Institute for Health and Care Excellance; [updated February 2019]. Available from: https://www.nice.org.uk/guidance/cg62/chapter/1-Guidance.
• 7. Walters A, McKinlay C, Middleton P, Harding JE, Crowther CA. Repeat doses of prenatal corticosteroids for women at risk of preterm birth for improving neonatal health outcomes. Cochrane Database of Systematic Reviews 2022, Issue 4. Art. No.: CD003935. DOI: 10.1002/14651858.CD003935.pub5