Yes, babies have a surprisingly active hormonal system that begins developing months before birth. By the end of the first trimester, a fetus is already producing several hormones on its own, and by the time a baby is born, dozens of hormones are at work regulating everything from blood sugar to brain development. Some of these hormones reach levels that rival or even exceed what you’d find in an adult.
Hormones Start Early in the Womb
A baby’s endocrine system doesn’t wait for birth to switch on. Between 10 and 17 weeks of gestation, the fetal pituitary gland is already releasing growth hormone, thyroid-stimulating hormone, and several other signaling hormones. The fetal adrenal glands become active around 9 to 12 weeks, producing steroid hormones that support development. And the pancreas begins making insulin as early as 13 weeks, helping the fetus manage its own blood sugar independently of the mother.
Before a baby’s own glands are fully online, the placenta acts as a bridge. Maternal thyroid hormones cross into the fetus during a critical window between roughly 12 and 20 weeks, before the baby’s thyroid is producing enough on its own. This early transfer is essential for normal brain development. The placenta also produces hormones that increase calcium flow from the mother’s bones to the fetus, which is so important for skeletal development that animals lacking this hormone die at birth with severe bone abnormalities.
The Cortisol Surge at Birth
Birth itself triggers one of the most dramatic hormonal events in a person’s life. For most of pregnancy, fetal cortisol (the primary stress hormone) stays relatively low, around 5 to 10 micrograms per milliliter. By 36 weeks, it climbs to about 20. In the final days before labor, it rises further to around 45. Then, during and immediately after delivery, cortisol spikes to roughly 200 micrograms per milliliter.
This isn’t a sign of distress in the harmful sense. That massive cortisol surge, along with a flood of other hormones, helps the newborn make the transition from life inside the womb to breathing air. It prepares the lungs, stabilizes blood pressure, and supports circulation during those critical first hours.
Mini-Puberty in the First Months
One of the most surprising hormonal events in infancy is something called “mini-puberty,” a temporary activation of reproductive hormones that happens in the first six months of life. In boys, testosterone levels are low at birth but rise quickly, peaking between one and three months at levels just below what you’d see in an adult man. By four to six months, testosterone drops back to undetectable levels and stays there until actual puberty years later.
In girls, estrogen levels are high at birth (carried over from the mother), drop in the first few days, then rise again about a week later as the baby’s own ovaries become active. Estrogen fluctuates and gradually decreases over the first two years. This mini-puberty drives real physical changes: testicular and penile growth in boys, and follicular development in girls. It’s completely normal and requires no treatment.
Visible Signs of Newborn Hormones
Some of the most noticeable effects of infant hormones are the ones that alarm new parents. About 70% of newborns develop some degree of breast swelling, caused by maternal estrogen still circulating in their system. As that estrogen drops after birth, it triggers the baby’s own pituitary gland to release prolactin, the same hormone that stimulates milk production in nursing mothers. Between 5% and 20% of newborns actually produce small amounts of breast milk, historically called “witch’s milk” because of old folklore that supernatural creatures would feed on it. The swelling and any milk production resolve on their own as hormone levels settle.
Neonatal acne, those small bumps that appear on a newborn’s face in the first weeks, is another visible result of hormonal activity. It typically clears without intervention as the baby’s hormone levels stabilize.
Thyroid Hormones and Brain Building
Few hormones matter more to a baby than thyroid hormones. In both the fetus and the newborn, thyroid hormones drive the fundamental architecture of the brain. They control when and where new brain cells form, how neurons migrate to their correct positions in the cerebral cortex, and how the brain’s wiring gets insulated with myelin (the fatty coating that allows nerve signals to travel quickly).
The process is remarkably precise. Thyroid hormones regulate the timing of gene activity in different brain regions, essentially telling specific genes when to turn on and off during development. They also control specialized cells that produce a protein called reelin, which acts like a scaffolding guide, directing neurons to migrate in the correct inside-out pattern as the cerebral cortex forms its layers. When thyroid hormone levels are too low during this window, the cortex develops with poorly defined layers, myelination is delayed and sparse, and lasting neurological problems can result. This is why newborn screening for thyroid function is one of the first tests performed after birth.
Growth Hormone and IGF-1
Growth hormone is the primary driver of childhood growth, and it’s active from the earliest months of life. It works mainly by stimulating the production of a secondary hormone called IGF-1, which directly promotes growth in bones, muscles, and the brain. Babies with higher IGF-1 levels at three months tend to gain more body length in the months that follow, likely through effects on the growth plates in their bones.
IGF-1 is important both before and after birth. Prenatally, it supports overall body size and brain growth. Postnatally, higher circulating levels correlate with faster growth in preterm and low-birth-weight infants, making it a key factor in how well smaller babies catch up.
Melatonin and the Sleep-Wake Cycle
If you’ve ever wondered why newborns have no concept of day and night, hormones are a big part of the answer. Newborns don’t produce melatonin, the hormone that signals darkness and promotes sleep, in any meaningful rhythm. A detectable melatonin cycle doesn’t emerge until around 9 weeks of age. Cortisol develops a daily rhythm at about 8 weeks, body temperature follows at around 11 weeks, and a consistent pattern of sleeping more at night than during the day typically isn’t established until 12 to 16 weeks.
Before these rhythms kick in, newborns rely partly on melatonin transferred through breast milk, which contains higher levels at night. This is one reason sleep experts recommend keeping nighttime feedings dim and quiet: it supports the gradual development of the baby’s own internal clock.
Oxytocin and Bonding
Babies produce oxytocin, often called the bonding hormone, and it plays a measurable role in how they connect with their parents. During skin-to-skin contact, oxytocin levels rise significantly in the infant, the mother, and the father. Parents with higher oxytocin levels during these interactions show more synchrony and responsiveness with their babies, and the effect runs both directions: the baby’s rising oxytocin reinforces the attachment cycle. Skin-to-skin contact also reduces stress hormones and anxiety responses in both parent and child, creating a feedback loop where closeness promotes calm, and calm promotes closeness.