Low birth weight (LBW) is defined as a birth weight <2500 g and frequently seen in cases of pre-term birth (PTB; <37 weeks gestation) and/or babies born small-for-gestational-age (SGA; <10th percentile for birth weight).
Attributable causes for SGA differ between developed and developing areas. Nutritional factors (low weight gain, low maternal BMI, short maternal stature) play a more significant role in the rural, developing areas. Lifestyle-related factors (e.g. cigarette smoking) as well as low weight gain and low maternal BMI account for 50% of SGA in developed countries.
PTB remains the largest single contributor to the infant mortality rate (IMR) and is associated with developmental deficiencies. SGA has a stronger association with still births and adult-onset diseases such as hypertension and diabetes, but only a modest effect on the IMR. In North America, 25-year trends for PTB are rising, while those for LBW and the IMR are falling.
Many areas of the developing world continue to show a relatively higher prevalence of both PTB and LBW-SGA, although also with decreases in IMR. Professor Kramer discusses the relative importance of ethnic and regional differences on the risks for LBW-SGA. He concludes that preventing LBW will have little effect on improving the IMR as this is driven more by PTB rates. The modifiable factors (maternal nutritional status, smoking and others) have more effect on SGA.