Material and Method: The data were obtained from the Prospective Cohort Study in Thai Children (PCTC). Three thousand five hundred twenty two pregnancies initiated the follow-up in the year 2000 at four districts across Thailand. The birthweight was retrieved from the Delivery Summary Sheet of the hospitals. The babies born in the hospital were weighed within the first day of life. The babies born at home were weighed within 3 days after birth by the research assistants. Only singleton live births were included in the present study.

Results: Three thousand three hundred twenty two live births with birthweight data, 8.6% were LBW. Maternal factors affecting LBW with high attributable fraction (AF) and moderate population attributable risk (PAR) were maternal extreme age (AF = 45.96, PAR = 16.24) and weight gain during pregnancy less than 10 kg. (AF = 40.12, PAR = 16.05). The factors with moderate AF and PAR were first and more than two parities (AF = 21.9, PAR = 15.51) and less consultation to health personnel (AF = 20.96, PAR = 16.98).

Conclusions: Improving nutritional status of pregnant women is a potential activity to reduce the incidence of LBW. Pregnant women with extreme age, first and more than parity two and less consultation to health personnel should also be closely followed-up.

Keywords: Low birthweight, Cohort study, Population-based, Attributable fraction, Population attributable risk