Puberty in girls: what changes, when it starts and when it ends

What changes during puberty?

Almost everything changes during puberty, though not all at once: breasts, height, bones, sleep and mood each shift on their own schedule. It's common for a girl, or the parent watching her, to wonder whether she is early, late or somehow off course. Most of the time she is right on her own track, and knowing the usual order of events makes the whole thing feel far less mysterious.

Breast development is one of the earliest signs, so a first period is a later step in the sequence, not the opening one. The NHS draws the typical starting window a little wider than the US range, at 8 to 14 (NHS, Early or delayed puberty). Signs that appear before age 8 may point to early puberty, which is worth raising with a pediatrician (a GP in the UK), who can say whether anything needs a closer look. In many cases the answer is simply that she is at the early end of normal.

Height follows a pattern that surprises many families. Girls grow fastest about 6 to 12 months before their first period, and then growth slows (ACOG Committee Opinion 651, 2015). A girl who seems to outgrow everything she owns in a single school year is probably working through exactly this phase. Underneath the visible growth, her skeleton is having its busiest years: Bailey and colleagues (1999) found that about a quarter of adult bone is laid down in the 2 years around the growth spurt. None of this can be seen from the outside, yet it is one of the most important jobs puberty does, and it is the reason food and movement matter so much at this stage.

Bodies change shape too, and many girls notice that before anything else. It helps to hear early that this is ordinary, and that talking about what a body can do, such as its strength, energy and skill, tends to feel better than talking about how it looks.

Feelings and sleep shift alongside the body. The body clock moves later in puberty, so teenagers feel sleepy later at night; a late bedtime is often biology rather than a choice, while school mornings stay exactly where they were. Moods can also run bigger and change faster than they did in childhood, and plenty of girls sail through with the usual ups and downs. Gentle, regular check-ins give a girl room to say when something feels heavier than a bad week, and low mood, anxiety or worry about food or body that lingers deserves a real conversation rather than a shrug of "just hormones".

Once periods begin, they become part of the picture of her overall health. ACOG treats the menstrual cycle as a vital sign, which is why questions about periods are a normal part of adolescent check-ups (ACOG Committee Opinion 651, 2015).

What helps a girl's body through these years?

Enough food, daily movement, steady sleep and an adult who listens are what help most, and none of them has to be perfect. Small habits that hold on most days do more than big plans that last a week. Here is where the evidence points, so you can pick a good place to start.

Food comes first, and iron deserves particular attention as growth and periods get under way. The US target is 15 mg of iron a day, and the UK figure is 14.8 mg (NIH Office of Dietary Supplements; National Diet and Nutrition Survey). Needs rise once periods start, and many teenage girls fall short. Regular meals that include iron-rich foods are a sensible first step, and a doctor can advise if you suspect the gap is wider than food can fill.

For movement, a sensible target is 60 minutes a day, including muscle- and bone-strengthening activity on 3 days a week (CDC, 2025). The CDC gives jumping and running as examples of bone-strengthening activity, so this can be as simple as a ball game, dancing or a skipping rope. The reason is worth sharing with her: the bone she builds now helps set her fracture risk after menopause (Weaver et al., 2016). The work she does today is a gift to her older self.

Girls' activity tends to drop steeply through adolescence, and only about 1 in 6 US high-school girls is active for 60 minutes every day (CDC Youth Risk Behavior Survey, 2023). Many families find that the activity she enjoys, with friends she likes, lasts longer than the one that looks best on paper.

Sleep is the third support. Teens do best on 8 to 10 hours a night (CDC, 2025), and a consistent routine matters more now than it did in childhood. A regular wind-down and a similar wake time across the week give her something steady to hold on to.

The last support is the simplest and sometimes the hardest: staying easy to talk to. A girl who knows she can mention a heavy period, a worry about her body or a rough patch at school, without it becoming a big event, is far more likely to speak up early. Talking about food in terms of energy and strength, rather than size, tends to keep those conversations easy.

Next steps

If you are a parent, a good place to start is simply noticing where your daughter is in the sequence and writing down the dates that matter, so you have them if a question comes up. From there, it's worth looking at the three everyday supports, food, movement and sleep, and choosing the one that feels easiest to nudge this month. One small change that sticks is worth more than an overhaul that fades. If you are the one going through puberty, you might pick a trusted adult and ask the question that has been on your mind, because the chances are high that it has a simple answer. If anything in the signs listed on this page sounds familiar, booking a visit with a pediatrician or GP is a calm, sensible step, and bringing your notes will make that conversation easier. The guides linked from this page go deeper on first periods, nutrition, training, sleep and health checks, so you can read only what is useful right now. Every girl moves through these years at her own pace and in her own way. Puberty is not one size fits all, so you and she get to decide what fits.