Objective To explore the correlation between the dynamic changes of biomarkers such as fat, protein, lactose, minerals, and sodium potassium ratio in breast milk of primiparous women and delayed lactation initiation, and determine corresponding warning values, providing scientific basis for early clinical intervention. Methods Convenience sampling was used to select 204 primiparous women who gave birth in a tertiary maternity and child specialist hospital from January 2023 to December 2024. Starting from after giving birth, breast milk samples were collected daily from 9-11am for 3 days. The content of fat, protein, lactose, and minerals in breast milk were measured using a fully automatic infrared breast milk analyzer. The sodium potassium content was measured using a fully automatic biochemical analyzer, and the sodium potassium ratio was calculated. At the same time, the start time of lactation was recorded in detail to determine if there was a delay in lactation. Results During the observation period of 204 primiparous women, the incidence of delayed lactation initiation was 28.43% (58/204). The composition of breast milk changed significantly 1-3 days postpartum, with an increase in fat, protein, lactose, and mineral content, and a decrease in sodium potassium ratio. ROC curve analysis showd that each component had predictive value for delayed lactation initiation. The AUC for fat prediction was 0.709-0.740, with a critical value of 0.311-1.620g/100g. The AUC for protein prediction was 0.700-0.808, with a critical value of 0.435-1.254g/100g. The AUC for lactose prediction was 0.777-0.803, with a critical value of 2.503-5.728g/100g. The AUC for mineral prediction was 0.771-0.854, with a critical value of 0.101-0.179g/100g. The predicted AUC for the sodium potassium ratio was 0.785-0.902, with a critical value of 1.870-2.776. Correlation analysis showed that within 1-3 days postpartum, the content of fat, protein, lactose, and minerals were negatively correlated with delayed lactation initiation (r range:〖CD*3/5〗 0.159-0.509, all P<0.05), and the sodium potassium ratio was positively correlated with it (r range:0.387-0.621, all P<0.001).Conclusion The content of fat, protein, lactose, and minerals in breast milk gradually increases from the first to the third day after delivery, and the sodium potassium ratio gradually decreases, which is related to delayed lactation initiation. When the fat content is below 0.311g/100g, the protein content is below 0.435g/100g, the lactose content is below 2.503g/100g, the mineral content is below 0.101g/100g, and the sodium potassium ratio is high at 2.776, it is necessary to be alert to the occurrence of delayed lactation initiation. Monitoring these biomarkers in breast milk on the first to third day postpartum is of great significance for early prediction of delayed lactation initiation and taking effective preventive measures