The activation of the hypothalamic-pituitary-gonadal axis (HPG) develops through three main moments: foetal life, first postnatal months (usually first 6 months of life) and finally during puberty. During the foetal period, there is a peak of gonadotropin secretion starting from the second trimester of gestation, and a subsequent suppression in the last weeks until birth, due to the negative feedback of placental estrogens. In male foetuses, LH levels exceed those of FSH. The foetal testis secretes testosterone and anti-müllerian hormone (AMH) from the 8th week of gestation and this is essential for masculinization. Formation of the active metabolite of testosterone, dihydrotestosterone, is required for the development of the prostate, penis and scrotum. Initial testicular development is intra-abdominal and the descent of the testes into the scrotum occurs in two phases. The first, transabdominal, phase is completed by 15 weeks of gestation. The second, inguinoscrotal, phase is usually completed by the end of the 35th week of gestation and this phase is androgen dependent. Testosterone levels are high in male foetuses between 10 and 20 weeks of gestational age, reaching adult values, and decrease thereafter towards term. In the same way LH and FSH levels decrease towards the end of gestation and are low at term in both sexes, due to the negative feedback of placental oestrogens.

A second post-natal HPG activation, also called "minipuberty", occurs at around 1 week of age, when placental hormones are cleared from the circulation. From the first weeks of life onwards there is a progressive increase in gonadotropins levels with a greater increase of FSH in females and of LH in males. During the "minipuberty" period gonadotropins levels have a peak at 3 months of life and a progressive exhaustion at around 6 months, except for FSH levels in females that can remain high even up to 3 or 4 years. Particularly in males, testosterone and androgen levels are associated with development and maturation of the reproductive system (penis and testis) as well as the androgynous cutaneous manifestations and a different neurobehavioral structure. Postnatal testosterone levels have been associated with male-type behaviour in 14-month-old infants, suggesting a role in neurobehavioral development.6 Recently, it has also been suggested how the increasing testosterone level during the first 6 months of life, as well as during puberty, translates into an increase of linear growth, regardless of the levels of growth hormone (GH) or insulin-like growth factor (IGF1).

Preterm birth does not seem to influence post-natal HPG activation, since that gonadotropin levels seem to raise up at the same time as term infants after the birth. The difference is that in preterm neonates these values last higher and longer than full-term newborns. According to the most recent longitudinal data, the post-natal activity of this axis declines at about the same in term neonates compared to premature infants with the same corrected gestational age, suggesting that the HPG activity is evolutionarily regulated. This higher increase of gonadotropin levels in premature males has been associated with a faster penile and testicular growth after birth compared to full-term boys. At last, in neonates small for gestational age (SGA) there are some evidence that post-natal FSH and testosterone levels are higher compared to infants with an adequate weight (AGA), although it is not known whether this correlates or have any influence on the catch-up growth of this subjects.

Understanding how "minipuberty" can influence linear growth, genitalia development and neurocognitive processes in preterm neonates has a significant scientific impact and could provide more information in understanding auxological and neurobehavioral developmental patterns of this population.