This is a prospective cross sectional observational study conducted in the Department of Neonatology at a tertiary care teaching hospital. The study aims to evaluate serum sodium levels in healthy late preterm and term breastfed neonates with excessive weight loss during the first postnatal week. Excessive neonatal weight loss is an early indicator of inadequate breastfeeding and may predispose to hypernatremia and dehydration.

Eligible neonates with gestational age greater than 34 weeks and birth weight between 2000 and 4000 grams who are receiving breast milk feeds will be screened by daily weight monitoring from birth until day 7 of life or discharge whichever occurs earlier. Babies with significant weight loss will undergo serum sodium and serum creatinine estimation as per routine unit protocol. No additional blood samples will be collected exclusively for research purposes. Maternal breast milk sodium estimation will be performed through routine laboratory services after obtaining written parental consent.

The primary objective is to determine the incidence of hypernatremia defined as serum sodium greater than or equal to 145 millimoles per liter among neonates with excessive weight loss. Secondary objectives include evaluation of the relationship between neonatal serum sodium levels, maternal breast milk sodium levels, serum creatinine levels, clinical features, and maternal and neonatal risk factors.

The study hypothesis is that excessive neonatal weight loss in the first postnatal week is associated with a higher incidence of hypernatremia and is related to maternal breast milk sodium levels and other clinical variables. The findings may support early identification of at risk neonates and help in developing evidence based screening strategies.