Objective The thymus plays a critical role in immune functioning. The objective of this study was to determine the thymic size in term low birth weight (LBW) newborns and to identify the relationship between thymic size and maternal and neonatal nutritional status.

Methods The study population included 44 randomly selected healthy term LBW (birth weight <2500 g) neonates and 71 gestational age-matched newborns weighing ⩾2500 g served as controls. Thymic dimensions including transverse diameter, longitudinal diameter, thymic index and thymic volume were measured sonographically on days 3–7 of age. Maternal nutritional parameters assessed were body mass index, mid-arm circumference and haemoglobin. Neonatal nutritional status was assessed by measuring birth weight, crown–heel length, head circumference, chest circumference, mid-arm circumference and triceps skin-fold thickness. Cord blood zinc and copper levels were estimated by atomic absorption spectrophotometry.

Results The thymus was nearly half the size in Indian babies compared with western newborns. All thymic dimensions, namely, transverse and longitudinal diameter, thymic index and thymic volume were further reduced in LBW infants compared with controls. Thymic size correlated with maternal body mass index and mid-arm circumference and neonatal anthropometry. Thymic dimensions showed significant correlation with cord blood zinc levels but not with copper levels.

Conclusion Thymic size is reduced in term LBW neonates and is linearly related to maternal and neonatal nutritional status and cord blood zinc levels. Small thymus in LBW infants may have potential significance for these babies with regard to their immunological functioning.