Pre-pregnancy underweight, inadequate gestational weight gain, and risk of low birth weight among a low-risk obstetric population: A community-based case-control study in Central Sri Lanka

Background: Low birth weight (LBW; <2,500 g) is a persistent public health concern globally and in Sri Lanka, where approximately 18% of newborns are affected, with important short- and long-term consequences. Maternal nutritional status, reflected by pre-pregnancy body mass index (BMI) and gestational weight gain (GWG), is hypothesised to influence foetal growth; however, community-based evidence on their independent effects in Sri Lanka is limited. This community-based case-control study assessed the independent associations of pre-pregnancy BMI (World Health Organization categories) and GWG (Institute of Medicine recommendations) with LBW among a low-risk obstetric population in the Central Province of Sri Lanka.Methods: A total of 450 singleton mother–infant pairs registered by ≤12 gestational weeks and followed through delivery (2020–2022) were included: 90 LBW cases and 360 normal birth weight controls (2,500–3,999 g). BMI was calculated from weight measured at/before 12 weeks; GWG was computed from weights at/before 12 weeks and at ≥37 weeks. Logistic regression estimated adjusted odds ratios (AOR) after adjusting for maternal age, parity, and antenatal anaemia.Results: Pre-pregnancy underweight was strongly associated with LBW (AOR 9.31, 95% CI 5.17–16.76; p < 0.001) and inadequate GWG independently increased LBW odds (AOR 2.71, 95% CI 1.41–5.22; p = 0.003). Overweight/obesity, excess GWG, maternal age, antenatal anaemia, and infant sex were not significantly associated.Conclusion: Pre-pregnancy underweight and inadequate GWG are independent, modifiable risk factors for LBW, supporting the need for strengthened preconception nutritional interventions and structured antenatal monitoring to reduce the LBW burden.