Hormones start remarkably early, well before birth. By 9 to 12 weeks of gestation, the fetal adrenal glands are already producing steroid hormones. From that point forward, hormone activity only increases, shaping development through infancy, childhood, and the dramatic changes of puberty.
Hormones in the Womb
The endocrine system is one of the first functional systems in a developing fetus. By 9 to 12 weeks of pregnancy, the fetal adrenal glands begin producing steroid hormones, including precursors to estrogen and testosterone. Between weeks 10 and 17, the pituitary gland (a pea-sized structure at the base of the brain that acts as a master controller) starts releasing a suite of hormones: growth hormone, thyroid-stimulating hormone, and the hormones that will later drive puberty.
The thyroid gland follows its own timeline. Early in pregnancy, a fetus depends entirely on thyroid hormones transferred from the mother through the placenta. This maternal supply is critical for brain development between weeks 12 and 20. The fetus doesn’t produce meaningful amounts of its own thyroid hormone until the second trimester.
The pancreas joins in around week 19, when fetal cells begin secreting insulin to help regulate blood sugar. Even cortisol, the body’s primary stress hormone, has a fetal chapter. While the adrenal glands are active early, they don’t produce cortisol in its final form until around 30 weeks of gestation.
Growth Hormone Surges at Birth
Newborns experience a burst of growth hormone immediately at birth that is far higher than anything seen later in childhood. In the first day of life, growth hormone levels can spike as high as 191 ng/ml, with five or six pulses of release within a six-hour window. These peaks drop quickly, becoming less pronounced within the first four days, and by one to two months of age, levels settle into ranges typical of older children.
One interesting quirk of early life: sleep doesn’t trigger growth hormone release in newborns the way it does in older kids and adults. That familiar link between deep sleep and growth hormone doesn’t kick in until around 3 months of age. Before that, feeding and insulin release are the main stimulators.
How Infants Build a Stress Response
The body’s stress hormone system, called the HPA axis, is present at birth but takes months to mature. In adults, cortisol follows a predictable daily rhythm: it peaks in the early morning and drops in the evening. Newborns don’t have this pattern yet. In fact, young infants often have higher cortisol in the evening than in the morning, the opposite of the adult rhythm.
By about one month, some infants show a two-peaked cortisol pattern, with rises in both the morning and afternoon, suggesting a transitional phase. The adult-like morning peak gradually emerges over the first several months, and a stable morning-to-evening ratio is typically in place by 6 to 9 months of age. This slow maturation mirrors the development of other circadian rhythms, like body temperature regulation.
When Babies Develop a Sleep-Wake Cycle
Melatonin, the hormone that signals nighttime and sleepiness, isn’t something newborns produce on their own. Circadian rhythm begins developing during fetal life and is initially driven by the mother’s hormonal signals. After birth, the infant’s internal clock starts responding to environmental light at around 10 days old, but actual melatonin production doesn’t emerge until 8 to 12 weeks of age in full-term infants. Preterm infants may take even longer.
Cortisol’s wake-promoting rhythm develops slightly earlier, around 8 weeks, followed by melatonin’s sleep-onset rhythm at approximately 9 weeks. This is roughly when many parents notice their baby starting to distinguish day from night more consistently.
Adrenarche: The Quiet Prelude to Puberty
Years before any visible signs of puberty appear, the adrenal glands ramp up production of a hormone called DHEA around age 8. This process, called adrenarche, is often the body’s first hormonal step toward maturation. It can cause subtle changes like body odor, oily skin, and the beginnings of pubic hair, but it’s separate from the main engine of puberty and doesn’t involve the sex organs. DHEA production continues rising from around age 8 all the way into the mid-20s.
How Puberty Starts
The hormonal cascade most people think of as “puberty starting” is gonadarche, the activation of the reproductive hormone system. This system actually ran briefly during fetal life and shortly after birth, then went dormant throughout childhood. At the onset of puberty, a network of neurons in the brain “wakes up” and begins releasing a signaling molecule called GnRH, which triggers the pituitary gland to release hormones that stimulate the ovaries or testes. The result is rising levels of estrogen or testosterone, and the physical changes that follow.
A protein called kisspeptin plays a central role in flipping this switch. The timing is controlled by an epigenetic mechanism, essentially a shift from genes being silenced to genes being activated, though the exact triggers that determine when this happens in any individual remain complex.
Typical Ages for Puberty
In boys, the first physical sign of puberty is testicular growth, which begins at an average age of 11.1 years, with a normal range of roughly 9 to 13.5. Voice deepening, a later milestone, occurs around age 13 on average.
In girls, breast development (thelarche) is typically the first sign, generally appearing between ages 8 and 13. Menarche, the first menstrual period, comes later. For girls born between 2000 and 2005, the average age of menarche was 11.9 years, compared to 12.5 years for those born between 1950 and 1969. A Harvard study found that the rate of early menarche (before age 11) nearly doubled across those generations, from 8.6% to 15.5%, with the shift more pronounced among racial minorities and lower-income populations.
When Puberty Is Considered Too Early
Precocious puberty is defined as the onset of secondary sexual characteristics before age 8 in girls and before age 9 in boys. This threshold, endorsed by major pediatric and endocrine societies, is the point at which further evaluation is typically recommended. Precocious puberty can be driven by premature activation of the brain’s GnRH system (central precocious puberty) or by hormone production from another source, such as an adrenal or ovarian issue (peripheral precocious puberty). The distinction matters because the causes and management differ significantly.