Infants born before their expected due dates have, according to Browne (2003), always been a concern to their families. Only relatively recently though has the clinical condition of prematurity, and the concerns relating to developmental outcome of premature infants become the subject of research. A relevant factor is the increasing number of infants surviving preterm birth or its accompanying complications. Provision of hi-tech neonatal care and remarkable changes in the pharmacological treatment of these infants have led to an increase in the survival rate of preterm children in the US, the UK and other countries. Consequently this has posed challenges for healthcare professionals to ensure the identification of the most appropriate intervention for these infants (Browne, 2003; Azzopardi and Mallaiah, 2004; Swamy, Østbye and Skjærven, 2008). Swamy, Østbye and Skjærven (2008) observe that preterm birth is not a rare condition and although more frequent in developing countries, its incidence has been steadily increasing in developed, industrialised societies due to major medical advances and the subsequent decrease of infant mortality rates. From a population-based longitudinal study, Swamy at el (2008) reported a decrease from 10% to 2% in the mortality of preterm infants (GA<37 weeks) born in 1967 and 2002 respectively in
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Norway (N=1.167.506). Stoelhorst et al (2005) reported an even greater decrease in the mortality of infants born before 32 weeks of gestation when they compared two Dutch cohorts, one from the 1980s and one from the 1990s (76% to 33%). The rise of preterm deliveries has been attributed to factors like the use of assisted reproduction techniques, the increased use of obstetric intervention such as caesarean sections, the higher rate of twin and multiple births, and the higher prevalence of alcohol and substance misuse in urban areas (Slattery and Morrison, 2002). In the review of Slattery and Morrison (2002) on the incidence of prematurity as defined by the GA criterion, it is stated that “preterm delivery varies from 6% to 15% of all deliveries depending on geographical and demographic features of the population” (p.1490). The authors do not specify the countries to which these specific figures refer. Exact statistics and estimations are, according to the same review, very difficult to ascertain since there are variations between countries. The population-based study of Swamy, Østbye and Skjærven (2008), which used a retrospective cohort of 1.1 million singleton births in Norway, revealed 5.6% and 4.7% prevalence of preterm birth (<32 weeks) for boys and girls respectively. According to Adams and Barfield (2008) these rates very much depend on the homogeneity of the population, the access to medical care, and how well developed the social “safety net” is within the country of the study.
Irrespective of these factors, there is a tendency observed for prematurity rates to have increased over the years. According to the perinatal statistics of the national campaign “March of Dimes” in the United States, in 2003, 1 in 8 babies (12.3% of live births) was born preterm. This rate has increased nearly 12% between 1993 and 2003. Of the above 12.3%, 2% were very preterm, i.e. less than 32 weeks of completed gestation, and 10.4% moderately preterm, i.e. between 32 and 36 weeks of gestation (March of Dimes, 2005).
The NHS Maternity Statistics (2006), as cited in Bliss (2008), state that for the 2005/2006 the following were perinatal statistics in England:
16 Table 1: Comparison of gestation periods in England 2005/06
Gestation period Number of deliveries % of all births
Under 24 weeks 550 0.09%
24-28 weeks 3,600 0.61%
29-34 weeks 14,900 2.51%
35-37 weeks 57,300 9.66%
38-42 weeks 513,000 86.45%
over 42 weeks 4,060 0.68%
Prematurity has been defined on the basis of the infant's birth weight or gestational age, with the most common estimation of the latter being calculated, till recently, from the first day of the mother’s last menstrual period (Witter, 1993). Birth weight was used to define prematurity in the past but it has, after the refinement of ultrasound methods, been abandoned to the more accepted measure of gestational age. That is because low birth weight could relate to intrauterine growth retardation (IUGR) and could, therefore, have different aetiology to that of preterm birth (Aylward, 2004; Swamy, Østbye and Skjærven, 2008). Based on this argument, an infant born prior to 37 weeks of gestation is considered premature based on the normal length of pregnancy i.e. 40 weeks plus/ minus 2 weeks (Mattison, 2003; Bliss, 2005; Jepsen, 2006;
Swamy, Østbye and Skjærven, 2008). Although there is no lower limit for this definition, Slattery and Morrison (2002) suggest that 23-24 weeks’ gestation is broadly accepted as one, corresponding to an average birth weight of 500g.
Preterm birth can be a very heterogeneous classification. This classification of preterm birth must be qualified according to criteria such as: mechanism that initiates labour e.g. spontaneous or “induced” by the obstetrician; severity of prematurity as defined by gestational age; etiologic factor e.g. infection or placental haemorrhage.
This will subsequently impact on designing research studies in this area (Mattison, 2003). With regards to spontaneous or induced preterm birth, Adams and Barfield (2008) stress that the differentiation is imperative as preterm rupture of membranes, the major causal mechanisms of spontaneous preterm birth, often relates to intrauterine infection and subsequent risk for sepsis, phenomena that have been associated with adverse neurological outcomes. This is, however, not necessarily the case in an induced preterm birth, with the same authors suggesting that infants who were delivered preterm spontaneously at a GA of 28 to 31 weeks, had a higher risk of
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having cerebral palsy than the infants delivered at the same GA following the induced, medical procedure.
Some of the most typical classifications are presented in the tables below.
Table 2: Classification of Prematurity based on Gestational Age
37- 42 weeks Full term 28- 37 week Preterm Under 28 weeks Extremely preterm Above 42 weeks Post term
(Jepsen, 2006; Vergara, 1993)
Table 3: Classification of Prematurity based on Birth Weight
Above 2500 gram Average birth weight 1500- 2500 gram Low birth weight 1000-<1500 gram Very low birth weight Under 1000 gram Extremely low birth weight
(Jepsen, 2006; Vergara, 1993)
Table 4: Classification of Prematurity based on the relationship between GA and BW
AGA: Between the 10th and 90th percentile for gestational age in body weight SGA: Below the 10th percentilefor gestational age in body weight LGA: Over the 90th percentilefor gestational age in body weight
(Vergara, 1993; Merck Source, 2007)
The 1960s were the starting point for the advances in neonatology and obstetrics leading to current improved viability of the foetus at 27 weeks of gestation or below.
This increase in viability rates was also importantly accompanied by a decrease in major handicaps, justifying the expenditure of large amounts of time, money, resources and specialisation for neonatal care (Allen, 1993). However, survival accompanied by a decrease in the incidence of severe impairments, does not itself ensure the preterm child’s typical development. There is a need for a more careful examination of all and not just severe, clinical manifestations of prematurity in order
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to gain a good understanding of its actual impact on the life of the child and its family (Browne, 2003; Allen, 1993). Of concern are, for example, results of studies suggesting that children born preterm present with, rather “subtle” problems which might however interfere with their learning. Deficient attention, visual-perceptual or fine motor problems, poor receptive or expressive language and hyperactivity are only some of the problems reported to hinder these children’s learning, and are presented in the relevant section of this literature review (Chapters V and VI).
What follows is a debate on what the appropriate criterion is when defining and studying prematurity. A presentation of the main points of the “GA or BW” debate was deemed necessary, as it has led to the decision of using GA as the main prematurity defining “criterion” when selecting specific studies to be included in this review.