By: 1 June 2011

Almost one in 10 women will experience preterm birth.1 Most preterm births (about 70%) are caused by spontaneous onset of labour before 37 completed weeks of gestation. It is not a single disease entity. It has multi-factorial causes and, in order to prevent preterm birth and concomitant short and long term morbidity, a fundamental understanding of its aetiology is essential. Midwives are vital in the provision of maternity care and it is therefore important they have an understanding of the causes and consequences of preterm birth, as well as factors that could be tackled to reduce it.

The cost of preterm birth is staggering. It is estimated at £2.9 billion per year to the NHS, with more than 90% of these costs in the immediate neonatal period. This is comparable to NHS spending on smoking or alcohol. Many socio-demographic characteristics are related to the risk of early birth, particularly ethnicity and environmental factors such as smoking, but none on their own will accurately predict who will labour early.

A large proportion of women who deliver early are low risk, with no previous history. Indeed there is evidence from several countries that rates of preterm birth are increasing, even in low risk women in their first pregnancy.2 The reasons for premature birth are complex, ranging from activation of the fetal hypothalamic pituitary access, infection/inflammation, decidual haemorrhage or ischemia, or uterine over-distension. Changes in assisted conception rates does not explain this increase alone.

Strategies to prevent preterm birth have been largely medical, including drugs to create uterine quiescence, either given prophylactically (progesterone) or in threatened preterm labour (nifedipine/atosiban) or surgical prophylactic measures, such as cervical cerclage.3 Maternal steroids are given to enhance fetal maturation, and this improves the lung function of many preterm infants. This approach is mostly reactionary, and a more holistic approach is required if we are really going to impact on this problem; these measures have clearly not reduced the number of preterm births over the years, nor the impact of its consequences.

A careful history may pick up factors that can be influenced. For example, smoking is known to be associated with preterm birth and reduction, or abstinence, will reduce it. Other important social factors include recreational drug use and domestic violence, both potentially reversible with the correct management.

A number of conditions are amenable to surgical intervention.4 Women who have had cervical surgery prior to their pregnancy have nearly double the risk of preterm birth. Women with previous medical abortions are also at higher risk, even if they have only had one procedure. Family history may also reveal factors amenable to intervention (such as cerclage). Drug use by the mother in a previous pregnancy, such as diethylstilbestrol (DES) can cause cervical incompetence in her female offspring. Women with uterine anomalies, such as uterine didelphies or unicornuate uterus, should also be referred, as they too have an increased risk of early birth. Early referral for obstetric opinion to consider cervical cerclage is essential.

An important easily reversible factor is a recurrent history of urinary tract infections. If women have had a previous early delivery, between 16 – 37 weeks their risk of a subsequent preterm birth is nearly doubled. The earlier the previous birth, the higher the risk. Risk also increases with the number of previous preterm births. Despite this, even with multiple risk factors, most women can achieve a good outcome.

Over the last decade fetal fibronectin and cervical scanning have revolutionised the ability to predict outcome. Transvaginal ultrasound cervical measurements can be used to predict early birth. Measurements less than 25mm are abnormal and cervical cerclage can also be used in these women to reduce risk by about 25%. When this scan is combined with a simple vaginal swab to measure fetal fibronectin (, prediction of preterm birth is greatly improved. Women who test negative for fibronectin (even with a short cervix) have a very low risk of a seriously early birth and can be appropriately reassured, even with multiple risk factors. Women who test positive are likely to deliver early (nearly 50% will deliver before 30 weeks if positive at 22 weeks in an asymptomatic women),5 and therefore surveillance and prophylactic strategies can be appropriately targeted in addition to support for these women.

The effectiveness of other interventions in high risk women, is currently being investigated in numerous trials. The role of vaginal or intramuscular progesterone in women with a short cervix looks promising.6 Women at very high risk can be targeted with steroids, given at optimal time immediately prior to delivery. Although abnormal vaginal flora such as bacterial vaginosis (BV) has been associated with a higher risk of early birth,7 unfortunately treating it does not seem to reduce risk and it is currently not recommended to routinely screen women for vaginal infections.8

Midwives should be aware of what to ask, who to refer, and the factors that are amenable to intervention. Midwives are key to identifying these and instigating the appropriate referrals and management. This is a very challenging area in desperate need of some focused evidence. We are getting very good at predicting, and we must learn the best ways to prevent. Midwives need to be actively involved in this research. The findings will enable midwives to provide optimal care for these women.


  1. UK Office for National Statistics. Preterm births, England and Wales, 2005. 2007. (last accessed 30 September 2010).[Full Text]
  2. Langhoff-Roos J, Kesmodel U, Jacobsson B, et al. Spontaneous preterm delivery in primiparous women at low risk in Denmark: population-based study. BMJ. 2006;332:937-939.[Abstract] [Full Text] 6497733
  3. Shennan AH, Bewley S. Why should preterm births be rising? BMJ. 2006;332:924-925.[Abstract]
  4. Simcox R, Shennan A. Cervical cerclage in the prevention of preterm birth. Best Pract Res Clin Obstet Gynaecol. 2007;21:831-42. [Abstract]
  5. Shennan A, Jones G, Hawken J, et al. Fetal fibronectin test predicts delivery before 30 weeks of gestation in high risk women, but increases anxiety. BJOG. 2005;112:293-298.[Abstract] [Full Text]
  6. da Fonseca EB, Bittar RE, Carvalho MH, et al. Prophylactic administration of progesterone by vaginal suppository to reduce the incidence of spontaneous preterm birth in women at increased risk: a randomized placebo-controlled double-blind study. Am J Obstet Gynecol. 2003;188:419-424.[Abstract]
  7. Shennan AH, Chandiramani M. Antibiotics for spontaneous pre term birth. BMJ. 2008;337:a3015.[Abstract]
  8. Simcox R, Sin WT, Seed PT, et al. Prophylactic antibiotics for the prevention of preterm birth in women at risk: a meta-analysis. Aust N Z J Obstet Gynaecol. 2007;47:368-377.[Abstract]