Introduction

The burden of low birth weight around the world

Human growth and development from conception to birth is a complex, highly regulated, and orchestrated process. Suboptimal fetal growth places the baby at increased risk of stillbirth and fetal distress in labour, while excessive growth can predispose to birth complications.

Low birth weight (LBW) has been defined by the World Health Organization (WHO) as a weight at birth of 2500 g or less, regardless of gestational age or sex. The selection of 2500 g was based on epi­demiological observations that babies were approximately 20 times more likely to die when born below this weight (1). However, this method of denoting small babies does not take into account the fact that LBW can occur because of infants being born preterm, or because of intrauterine growth restriction (IUGR), or both (2). It is essential to differentiate these conditions if evidence-based strategies are to be appropriately targeted. This threshold also fails to recognize that there are physiological differences between the sexes, with girls being lighter than boys at every gestational age; however, boys experience worse survival and perinatal outcomes in most populations. Finally, LBW is also a non-specific marker of several potential adverse exposures. Therefore the value of LBW to monitor and compare the state of peri­natal health between countries or over time has been queried (2).

In order to improve the definition and detection of babies with suboptimal growth, an expert committee from the WHO defined small for gestational age (SGA) as a birth weight less than the tenth percentile when compared to a reference for gestational age and sex (3). The terms average for gestational age (AGA, 50th centile) and large for gestational age (LGA, 90th centile) are also used. It is im­portant to note that SGA is not the same as IUGR—which is defined as failure to reach growth potential.

The WHO estimated that 15.5% of all births, equating to more than 20 million infants, were born with LBW in 2004 (1). The estimate for the number of babies born with LBW in low- and middle-income countries in 2010 was very similar, 18 million babies (4); however, as the total number of babies born continues to increase, even allowing for the uncertainly in these estimates, the rate of LBW is likely to be falling. In 2010, it was calculated that 59% of LBW babies were born at term (after 37 completed weeks of gestation) and 41% were preterm.

Evidence-based interventions to reduce mortality from preterm birth exist, but if inappropriately administered there is also a po­tential for harm. It is therefore essential to differentiate babies that are preterm from those that are term but growth restricted before birth. An example of the importance of correctly recognizing pre­term birth was inadvertently demonstrated in the ACT trial, a large cluster-randomized controlled trial to assess the efficacy of ante­natal corticosteroids to reduce deaths from prematurity in Africa, Latin America, and South Asia. Antenatal corticosteroid adminis­tration has been a mainstay of obstetric management of preterm labour for decades, with evidence from a Cochrane meta-analysis to support survival benefit when given to mothers in preterm la­bour at less than 34 weeks of gestation. In the ACT trial, staff in the intervention sites were trained to identify and administer ster­oids to women likely to deliver with either a birth weight below the fifth centile or gestational age less than 36 weeks, while in the con­trol arm no additional interventions were introduced. The trial ran for 18 months and during this time 51,523 women delivered in the control sites, and 48,219 in the intervention sites. The result, how­ever, was surprising, with an increased risk of perinatal death in the intervention group (relative risk (RR) 1.12; 95% confidence interval (CI) 1.02-1.22) (5). Correct differentiation of preterm from term but small babies is essential if interventions are to be appropriately given and outcomes improved.