Statistical analysis The sample number (10 breast-feeding and 10 not-breastfeeding mothers) can detect with 80% power a difference of 18% between ghrelin values after breastfeeding (α=0.05, β=0.2). Statistics will be performed using the SPSS release 12.0.1 software. Changes in hormone concentrations will be compared with repeated measures ANOVA. A possible association between parameters will be tested using linear regression analysis.