Gonadotropins which are high in the middle of the fetal life are measured to be considerably low in the cord blood and estrogen is found to be high in the cord blood. Gonodotropins are supressed by estrogen. After delivery, the hypothalamo-pituitary-gonadal axis is activated when estrogen is eliminated and a hormone profile which reaches pubertal levels is established. These changes are called mini puberty. In boys, long-term testicular functions and sperm production are regulated with mini puberty and mini puberty contributes to masculinization of the brain. The role of mini puberty in female newborns is not known. Central hypogonadism, Turner syndrome and ovarian hyperstimulation in preterm babies may be diagnosed with evaluation of mini puberty. In this article, mini puberty and related problems were reviewed and the importance of this issue was emphasized.