The term ‘adverse pregnancy outcomes’ usually refers to low weight at birth, i.e. to newborns that weigh below 2500 grams. Among these low-weight infants, the worst-off group includes those weighing below 1500 grams, who were also delivered pre-term (i.e. before the 37th week of gestation). All these babies face higher neonatal and infant mortality, as well as worse health during childhood and adulthood. Conditions such as growth retardation, neurological problems, hypertension, stroke, coronary heart disease, etc. have been linked to low weight at birth. Another adverse outcome is ‘heavy birth weight’ (i.e. weighing above 4500 grams), as it is linked to both complications during delivery and higher morbidity (for instance, diabetes and heart disease).
The World Health Organisation (2004) estimated that in 2000 prevalence of low birth weight was around 15.5% worldwide. Although the incidence is lower in developed countries, around 7% in 2000, the majority of neonatal and infant deaths are observed among low birth weight and preterm births. Hence, reducing the number of low birth weight has been outlined as a major goal in ‘A World Fit for Children’, the Declaration and Plan of Action adopted by the United Nations General Assembly Special Session on Children in 2002, in order to achieve a decline in infant mortality both in developed and developing countries.
Low, heavy birth weight and prematurity pose a significant burden on health care systems across countries, since they are linked to complications during delivery and to substantial costs to keep babies both alive early on and healthy during infancy, childhood and adulthood. In a nutshell, identifying modifiable factors linked to low birth weight and adverse pregnancy outcomes would not only decrease mortality and morbidity among infants but, potentially, would help reducing health care expenditure.
An important set of modifiable factors proven to significantly affect chances of low birth weight are socio-economic conditions. Recent analyses of vital registration data on 828,975 births of singletons occurring in Hong-Kong SAR between 1995 and 2009 reconfirmed their importance: a strong socio-economic gradient is apparent, more marked among low, rather than heavy birth weight babies. More specifically, lower occupational class and economic inactivity/unemployment of the father are significant predictors of having a birth below 2500 grams; the effect is even more pronounced for births below 1500 grams. Paternal educational attainment has an independent protective effect against low birth weight; having some tertiary education additionally protects against having a heavy birth weight baby, too. Living in public housing represents in Hong Kong SAR another dimension of material deprivation predicting adverse pregnancy outcomes. Whereas public housing and occupational class of the father reflect income, higher educational attainment has an additional property: more educated persons usually have a better knowledge of prevention and risky health behaviours. Not least, they also tend to make more efficient use of health care systems.
Finally, the results of the above-mentioned study confirm a significantly greater propensity to have a very low/low birth weight baby among unmarried women. Single motherhood reflects hardship up to a point, but also psychosocial stress, particularly in countries where such behaviour still represents a deviation from established norms. Analyses of Greek birth weight data for 2006 also indicate a strong negative association between extramarital status of a birth and adverse pregnancy outcomes, in a country where births outside marriage are still rather scarce.
The socio-economic gradient in birth weight emphasizes the need for implementing effective policies supporting disadvantaged population groups. The strong negative association between single motherhood and low birth weight found in such different contexts (Greece – Hong Kong) is especially worrying. In most European countries births outside the wedlock are on the rise, having reached levels as high as 50% or more in Western Europe and Scandinavia. Levels in countries such as Greece are low, around 5.3% in 2006. Anyways, margins for an increase are considerable and highlight the necessity of devising measures targeting this particularly vulnerable group of women.