We all remember going through puberty.

Your limbs might have felt gangly and awkward, and your body probably changed in some weird ways, from new body hair to acne and wild mood swings. But what if I told you that that wasn’t even the first time you went through puberty?

In their first weeks and months of life, babies go through a phase complete with surging hormones, and sometimes their side effects, that - just like puberty in teens - help to prepare their body for the rest of their life. This video is puberty 101, but for baby’s first puberty. [♪ INTRO] Now, regular puberty is important because it marks the time that bodies become reproductively mature. But babies don’t need to have babies - not yet, anyway.

So that begs the question: Why would a baby need to go through anything like puberty? First of all, the mini-puberty experienced by babies isn’t just about reproductive hormones. Cortisol, the stress hormone, is also really high in newborns.

Birth really is a stressful event for all parties involved! In addition to being the fight-or-flight hormone, cortisol helps to promote the final maturation of the lungs that infants need, so they’re ready to breathe air. It also revs their metabolism up higher so the baby can start to generate some of their own body heat, and dials down some less-essential functions, like growing hair.

More on that later. But we are talking about puberty, and yes, reproductive hormones do surge too. Let’s start with testosterone.

Testosterone begins to rise as early as a few weeks after birth and remains elevated until around six months, when it declines to normal, pre-puberty levels. And the testosterone levels in two month old infant males have been recorded to be as high as normal levels in adult males! This mini-puberty testosterone increase is triggered by the hypothalamic-pituitary-gonadal, or HPG, axis.

It’s the circuit that connects your brain, your pituitary gland, and your gonads, and it’s responsible for telling your reproductive organs to make reproductive hormones. In the lab, we can measure the hormones from the pituitary gland, like luteinizing hormone and follicle stimulating hormone, or we can measure those reproductive hormones directly from the gonads. Those hormones are even useful while the baby is in utero, since they help with tissue differentiation, but that reproductive circuitry is otherwise turned off.

After a male infant is born, the HPG axis is free from the high levels of placental estrogen present in the parent’s womb that were keeping it inhibited, which is why we see the early surge of testosterone in the first weeks after birth. It’s kind of a test-run for the system to work on its own. The HPG axis of female infants also kicks on, but the pattern of hormone release, timing, and duration is a little murkier.

Pituitary hormones and estrogen do increase, but how much seems to vary pretty dramatically person to person. Luckily, besides blood samples, hormones can be measured in saliva or urine, which babies make a lot of! That still leaves the question of what all this puberty business is for, since the reproductive organs aren’t actually ready for doing any reproducing in a tiny little baby.

Well, similar to the hormone exposure received during embryonic development, this mini-puberty has something to do with helping the body get ready for being a person: differentiating cells, turning on organ systems, et cetera. Hormones rarely do just one thing, after all - they’re kinda like a multi-purpose tool. The testosterone surge in infant males has been correlated with metabolism, language skill development, and possibly even their adult fertility.

It’s unclear if estrogen levels in infant females correlate to growth or fertility, but we do know that female preterm babies can experience more extreme hormone levels, so that’s at least another clue that there’s a developmental connection. To be honest, we still don’t have a complete understanding of why this baby puberty surge happens at all, let alone what makes it shut down again until the bigger, teenage puberty. We do know, though, that just like teen puberty, baby puberty can bring with it some of the same physical changes you’d see in adults’ hormone shifts, including one that happens way after teenage puberty - hair loss.

Some babies, even those born with full heads of hair, experience thinning and hair loss when they’re just a few months old, and the reason might be related to that cortisol surge we’ve mentioned. Cortisol tells your hair follicles to cut it out with the less-than-essential hair-growing. So it isn’t until after cortisol levels lower that regular growth can re-start, which can lead to a sort of baby-pattern baldness, if you will.

What’s wilder still, sometimes the fresh baby hair that grows back can be a different color, or texture than it was before. The surge in baby testosterone does impact genital development in male infants, but it’s usually nothing too noticeable. The changes are mostly in the cells inside the testes.

While sperm aren’t produced until teenage puberty, their precursors do get a boost from minipuberty. There have been reported cases of breast development and some vaginal bleeding in female infants born pre-term, possibly due to the decrease in estrogen levels they experienced when separated from their parent’s hormone supply. But in all of those cases, the symptoms resolved on their own in a matter of months.

Hormone shifts, changing body hair, and sudden growth are the most classic signs associated with teenage puberty, so it’s kinda funny to think that every acne-prone and wildly moody teenager went through something like the puberty they’re dealing with already. In any case, the next time you see a crying baby, have a little sympathy - puberty can be rough, even the mini-kind. [♪ OUTRO] Thanks for watching this SciShow episode. And if you want to know more about the quirks of how we humans grow up, check out this video from our sister channel SciShow Psych, on why we have such long childhoods.