Premature birth statistics

Definitions

•   Preterm: born before 37 weeks or at 259 days of gestation [1]
•   Moderately preterm: 33–36 completed weeks of gestation [2]
•   Very preterm: < 32 weeks [2]
•   Extremely preterm: < 28 weeks [2]
•   Low birthweight baby: weighs less than 2,500 g (5 lb 8 oz), regardless of gestational age at birth [3]
•   Very low birthweight baby: weighs less than 1,500 g (3.0 lb 5 oz) [3]
•   Extremely low birthweight baby: weighs less than 1,000 g (2 lb 3 oz) [3]
•   Neonatal mortality: deaths occurring during the first four weeks after birth [4]
•   Perinatal death: deaths in the first week of life and stillbirths [4, 5]

Incidence - in England and Wales

Nearly 8 percent (1 in 13) of live births are born preterm [6].
89 percent of singleton babies born extremely preterm are of very low birthweight, whereas fewer than 1 percent of those born at term are.
93 percent of preterm births occur after 28 weeks of gestation, but 6 percent occur between 22 and 27 weeks, and just under 1% occur before 22 weeks.
Approximately 50 percent of twin pregnancies are preterm [6, 7]
Preterm births occur in 5–10 percent of all births in resource-rich countries. In recent years, the incidence seems to have increased in some countries, particularly in the USA, where it was nearly 13 percent in 2005 and accounted for as many as 75 percent of perinatal deaths [7].

Causes

The following are the main causes of a preterm birth:
•   Spontaneous labour with intact membranes [8].
•   Preterm premature rupture of the membranes (PPROM) [8].
•   Labour induction or caesarean delivery. Around a quarter of mothers have complications including eclampsia, placental abruption or infection.

Survival rates

Research conducted in 1995 on babies born before 27 weeks of gestation in the UK and Ireland (the EPICure study) showed that only 7 percent of those born at 22 weeks survived, 42 percent of those born at 24 weeks survived and 78 percent of those born at 26 weeks survived [9].

Clinical outcomes

When the surviving children born before 27 weeks in the 1995 EPICure study were followed up at age 6 years, a high level of disability was found [10]:
•   22 percent had severe disability (defined as cerebral palsy but not walking, low cognitive scores, blindness, profound hearing loss)
•   24 percent had moderate disability (defined as cerebral palsy but walking, IQ/cognitive scores in the special needs range, a lesser degree of visual or hearing impairment)
•   34 percent had mild disability (defined as low IQ/cognitive score, squint or refractive error, requiring glasses)
When the same children were reassessed in middle childhood (aged 11 years), the investigators found that 45 percent had serious cognitive impairment. The following academic attainment was found [11]:
•   They had significantly lower scores for cognitive ability, reading and mathematics and 13 percent attended special schools.
•   57 percent of those in mainstream schools had special educational needs (SENs).
•   Those who entered school an academic year early due to preterm birth had similar academic attainment but required more SEN support

Sources
1. Beck S, Wojdyla D, Say L, et al. The worldwide incidence of preterm birth: a systematic review of maternal mortality and morbidity. Bulletin of the World Health Organization 2010;88(1):31–8.
2. Lawn JE, Gravett MG, Nunes TM, et al. Global report on preterm birth and stillbirth (1 of 7): definitions, description of the burden and opportunities to improve data. BMC Pregnancy and Childbirth 2010;10(Suppl 1):S1.
3. Small for gestational age (Wikipedia entry). Available at: http://en.wikipedia.org/wiki/Small_for_gestational_age (accessed 25 November 2012).
4. World Health Organization. Neonatal and Perinatal Mortality. Country, Regional and Global Estimates. Geneva: WHO; 2006. Available at: http://apps.who.int/iris/bitstream/10665/43444/1/9241563206_eng.pdf (accessed 26 November 2012).
5. World Health Organization. Maternal and perinatal health. Available at www.who.int/maternal_child_adolescent/topics/maternal/maternal_perinatal/en/ (2008, accessed 25 November 2012).
6. Office for National Statistics. Preterm births, preterm births data, press release based on 2005 data. Available at: www.ons.gov.uk/ons/publications/re-reference-tables.html?edition=tcm%3A77-50818 (2007, accessed 26 November 2012).
7. Ananth CV and Vintzileos AM. Epidemiology of preterm birth and its clinical subtypes. Journal of Maternal-Fetal and Neonatal Medicine 2006;19(12):773–82.
8. Goldenberg RL, Culhane JF, Iams JD, et al. Epidemiology and causes of preterm birth. Lancet 2008;371(9606):75–84.
9. EPICure 1995. EPICure - The first study. Available at: www.epicure.ac.uk/epicure-1995/ (accessed 25 November 2012).
10. Marlow N, Wolke D, Bracewell MA, et al. Neurologic and developmental disability at six years of age after extremely preterm birth. New England Journal of Medicine 2005;352(1):9–19.
11. Johnson S, Hennessy EM, Smith R, et al. Academic attainment and special educational needs in extremely preterm children at 11 years of age: the EPICure study. Archives of Disease in Childhood. Fetal and Neonatal Edition 2009;94(4):F283–9.