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The Epidemiology of Low BirthweightMichael S. Kramer 

This paper reviews the epidemiology of low birthweight (LBW), i.e.

a weight at birth <2,500 g. Because birthweight is determined by boththe duration of gestation and the rate of fetal growth, LBW can arisebecause an infant is born too soon (preterm birth) and/or too small forhis/her gestational age (SGA; see fig. 1) [1]. This distinction is importantin understanding recent temporal trends in developed countries, whichshow a rising incidence of preterm birth [2] but a decline in SGA birthdue to an increase in the weight of infants born at term [3].

Preterm birth has been estimated to affect nearly 10% of all birthsworldwide, 85% of whom are concentrated in Africa and Asia. SGA birth

affects over 20% of the world births, with 95% occurring in Africa andAsia. It remains unclear the extent to which population differences inbirthweight for gestational age are physiologic (‘normal’) vs. pathologic.

Preterm birth is the world’s leading cause of infant mortality [4].Although SGA infants born at term are also at somewhat increased risk ofinfant death, much of the recent literature focuses on long-term associa-tions with chronic diseases of adulthood, including hypertension, type 2diabetes, and coronary heart disease [5].

The major known causes of preterm birth include multiple birth,

genitourinary tract infection, pregnancy-induced hypertension/pre-eclampsia, low maternal pre-pregnancy body mass, short cervix, cigarettesmoking, placental abruption, prolonged standing and lifting at work,and cocaine use during pregnancy [1]. Early labor induction or prelaborcesarean delivery to reduce risks of stillbirth and/or maternal morbiditydue to pregnancy complications is making an increasingly large contribu-tion to the rise in preterm birth in high- and middle-income countries.The major causes of SGA birth include maternal cigarette smoking, lowgestational weight, low maternal pre-pregnancy body mass, short stature,

primiparity, pregnancy-induced hypertension/pre-eclampsia, malaria,congenital anomalies, other genetic factors, and alcohol/drug use [1].Effective interventions to prevent preterm birth are limited to inten-

sive counseling of pregnant women to reduce their cigarette smoking andprogesterone treatment of women with short cervix or prior history of

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preterm birth. Counseling to reduce cigarette smoking is also effective inreducing the risk of SGA birth, as are balanced energy/protein supple-mentation during pregnancy and malaria prophylaxis (in endemic areas).

Many countries in the world have succeeded in lowering their rates

of infant mortality. To my knowledge, however, none has succeeded indoing so by reducing its rate of LBW. Preventing LBW, preterm birth, andSGA birth are laudable, but thus far elusive, public health goals. Achievingthose goals will require far more research.

References

  1 Kramer M: Determinants of low birth weight: methodological assessment and meta-analysis. Bull WHO 1987;65:663–737.

  2 Joseph KS, Kramer MS, Marcoux S, et al: Determinants of preterm birth rates

in Canada from 1981 through 1983 and from 1992 through 1994. N Engl J Med1998;339:1434–1439.

  3 Kramer MS, Morin I, Yang H, et al: Why are babies getting bigger? Temporal trendsin fetal growth and its determinants. J Pediatr 2002;141:538–542.

  4 Howson CP, Merialdi M, Lawn JE, et al: March of Dimes White Paper on PretermBirth: The Global and Regional Toll. White Plains, March of Dimes Foundation,2009.

  5 Barker DJP: The developmental origins of chronic adult disease. Acta Paediatr Suppl2004;446:26–33.

Fig. 1.  Venn diagram indicating the relationship between LBW, SGA birth,and preterm birth (PTB).

LBWSGA PTB