What happens in puberty?

Before puberty, biological gender hardly has any effect on a child's physical stature, facial appearance or voice. These differences are usually only recognizable by external sex organs, although they are not fully developed. That changes at puberty.

It is still not clear what actually triggers puberty. Girls start puberty at around age nine. Boys don’t start until they’re around eleven. At that point the brain sends signals to the adrenal glands to start producing sex hormones. One of the things that the hormones are responsible for is the development of the gonads (testicles and ovaries). Once they have matured, they take over the production of the sex hormones and release them into the bloodstream in larger amounts.

Puberty ends in stages: Girls reach sexual maturity by the age of 14 and boys do by the age of 16, in some cases even earlier. In other words: Girls are then able to get pregnant and give birth, and boys are able to father a child. Girls’ bodies normally stop growing around the age of 16. Boys have usually finished growing by the time they’re 19. But it can take up to the early twenties for the last physical changes to happen. This can even take a little longer in boys.

The increase in the amount of sex hormones at the start of puberty triggers a growth spurt. Boys can then grow up to eight centimeters within one year. Because girls start puberty earlier than boys do, this growth spurt also tends to start first in girls. So they are often taller on average than boys of their age for a while.

Once the gonads start making the hormones, the transition to adulthood is mainly regulated by the estrogen hormones made in girls' ovaries and the testosterone made in boys' testicles. That affects growth: The increased levels of estrogen in the blood slows further growth in girls and they stop growing before boys do. So women are shorter than men on average.

The larynx also grows during puberty. The bigger it gets, the deeper it makes the voice. Because the larynx grows larger in boys than it does in girls, men's voices are typically lower than women's. While the voice box is still growing, the voice may sound shrill or croaky, and sometimes jump between high and low sounds. This is what people mean when they say someone's voice is “breaking.” It is generally more noticeable in boys than it is in girls.

The sex organs mature during puberty. They gradually develop all functions that are needed for pleasurable sex and having children.

The external sex organs change: The penis gets bigger, and the labia and clitoris grow. The internal sex organs mature as well. The testicles start to produce semen (which contains sperm cells). Boys start to ejaculate – often while they’re asleep (also known as "wet dreams"). In girls, the ovaries prepare to release one viable egg cell per month (ovulation). The womb (uterus) becomes an organ that readies each month for a possible pregnancy. This process is known as the menstrual cycle. The first period (menstruation) typically arrives when girls are about 13 years old – but is also sometimes earlier or later. Even if the first periods are irregular, it is possible to become pregnant from that time on. But it often takes some time for everything to have adjusted so that a (fertilized) egg cell can attach to the lining of the womb.

In addition to the internal and external sex organs, there are also secondary sex characteristics that develop during this time. One of these characteristics is the female breast. In girls, special "milk glands" (called mammary glands) start to grow during puberty, and more body fat is stored in the breast tissue. This causes breasts to gradually develop. The female hormones also cause the skin in other parts of the body to be “padded out” with more fatty tissue.

The proportions of the skeleton are different in men and women, so their general build is also different. Women tend to have a narrower waist and wider hips, while men have broader shoulders and narrower hips. But body shapes vary greatly and are not determined by biological sex alone. Many other factors can play a role, including sports, diet and your genes.

One of the other secondary sex characteristics is hair growing on some parts of the body: During puberty, hair starts to grow in the genital area and in the armpits that is typically thicker, darker and coarser than the hair that grows on your head. It can get up to several centimeters long. Boys’ pubic hair grows more up toward the belly button. The male sex hormone testosterone causes hair to start growing on their face, too. Hair on the rest of the body may become darker and thicker, depending on your skin type and genes. In men, it grows on their legs, forearms and chest, and sometimes also on their belly and back later on. In women it usually only starts growing on their legs.

Hormones cause the skin to produce more sebum (oil), which can lead to acne. Once hormone levels have settled down, typically in the early twenties, the acne usually goes away on its own.

Teenagers experience many highs and lows during puberty, and may often fell like they are on a roller coaster of emotions. One cause of this is probably processes of change occurring in the brain. Experts suspect that it is especially the parts of the brain that control how we make decisions that are developing. New connections are also created to the parts of the brain that are related to our emotions. During this phase of "restructuring," teenagers may then follow their impulses and whims more often than children or adults. They take more risks in some situations and experience sudden eruptions of emotion.

These changes make some teenagers feel uneasy. It is not unusual for conflict to arise with people they are in close contact with, like parents or teachers. And their acceptance of the new and unexpected may also help them develop completely new interests, hobbies and valuable friendships.

During puberty, the challenges, duties and expectations that others place on teenagers also change. For instance, they are supposed to finish up their schooling, choose an occupation, handle money responsibly, find friends their parents find appropriate and find a partner – or not yet. At the same time, they are confronted with unrealistic "role models" in social media – regardless of whether it's about looks, money, athleticism or being cool. For many teenagers it isn't easy to cope with all of that, and not everyone has the support they need. It takes time to find your own path.

This emotional turmoil is intensified even more by romantic feelings, longings and sexual desires, even if the physical maturation process is nearly finished or complete. Dealing with your own gender, sexuality and societal rules can pose many new questions: What does being a man or a woman mean to me, and do I see myself in the conventional roles? Does my biological sex feel right to me? What excites me sexually? Do I want to have a partner for sex, or is that not (yet) important to me? What do love and relationships really mean to me? It can take years to find the right answers. Some desires and preferences can also change later in life or first be discovered then.