The physical changes that occur during puberty are usually marked by distinct stages of development.

The stages are known as Tanner stages, named after Professor James Tanner, the child development expert who first identified them.

The Tanner stages give average ages of development, although there can be significant variation among children and teenagers. You therefore shouldn't worry if you reach a stage of puberty before or after your friends do.

Tanner stage one

Tanner stage one describes the body before the onset of puberty. This period is sometimes referred to as pre-pubertal.

Tanner stage one isn't associated with any particular ages, and there aren't any significant physical changes in girls or boys.

Tanner stage two

In girls:

• usually occurs at around 11 years of age
• breast buds develop – they're often very tender during the early growth stages (this is normal); one breast bud may also start to develop many months before the other one – again this is normal and the breast tissue will even up over the course of puberty
• your areola (area of skin surrounding the nipple) will begin to swell
• pubic hair will start to develop along the labia (lips of the entrance to the vagina)
• the womb becomes larger in response to oestrogen
• you'll grow taller by 7-8cm (2.8-3.2 inches) a year

In boys:

• usually begins at about 12 years of age
• your scrotum (the pouch containing the testes) will begin to thin and redden; your testicles will increase in size
• fine pubic hair will start to appear at the base of your penis

Tanner stage three

In girls:

• usually occurs after the age of 12
• the tissue beneath the areola continues to grow and spread out to provide the fullness of your breast; you may need to buy your first bra
• your pubic hair will become coarser and curlier and you'll begin to grow underarm hair
• you may develop spots (acne, see below) on your face and back
• you'll grow taller by an average of 8cm (3.2 inches) a year – the highest growth rate

In boys:

• usually occurs after the age of 13
• your penis will grow and lengthen, and your testicles will continue to grow
• your pubic hair will become thicker and curlier, spreading to the soft mound of skin above your genitals
• your breasts could swell slightly due to the growth of breast tissue; about a third of teenage boys have some breast tissue growth which usually settles down after a few years (read more about 'man-boobs')
• you may begin to have 'wet dreams' – involuntary ejaculations of semen during sleep
• your voice should 'break' (the pitch and tone of your voice may start to suddenly change for short periods of time)
• the size of your muscles will increase, and you will grow taller by 7-8cm (2.8-3.2 inches) a year

Tanner stage four

In girls:

• usually occurs at the age of 13
• your breasts slowly develop into a more adult shape, with your nipple and areola swelling to produce a secondary mound above the level of the breast (this will disappear after the rest of your breast has developed)
• your pubic hair will start to look more adult-like in appearance but won't have spread to your inner thighs
• you'll usually have your first period and you should start having regular periods in 6-12 months of your first one (around 10% of girls start their periods during stage five)  
• your growth rate will begin to slow down, growing taller by an average of 7cm (2.8 inches) a year

In boys:

• usually occurs at around 14 years of age
• your penis and testicles will continue to grow, and your scrotum will become darker
• your pubic hair will appear more adult-like, but won't have spread to your inner thighs
• you should start growing underarm hair
• your voice will change permanently
• you may develop acne

Tanner stage five (final stage)

In girls:

• usually occurs at just over 14 years of age
• your breast becomes adult-like in shape
• your pubic hair should spread to your inner thigh
• your genitals should have fully developed by the end of this stage
• you'll have stopped growing and reached your adult height 1-2 years after your periods started; from the age your periods start, you'll have another 5-7.5cm (2-3 inches) of growth in height

In boys:

• usually starts at about 15 years of age
• your genitals will look like an adult’s, and pubic hair will spread to your inner thigh
• you'll begin to grow facial hair and may have to start shaving
• your growth should slow down and you should stop growing at around 16 years of age (but your muscles may continue to grow)
• most boys will have reached full adult maturity by 18 years of age

Acne

During puberty, your body becomes more sensitive to the hormone testosterone, which is present in both boys and girls. Testosterone causes small glands in your skin to produce too much oil (sebum).

Dead skin can also block the opening of hair follicles (the small tubes in your skin that hold a hair in place). The sebum can build up behind the blocked follicle, which can cause spots (blackheads or whiteheads) to develop.

Hormonal changes also alter the levels of acid in your skin, encouraging the growth of bacteria. If bacteria infect a blocked hair follicle, a deeper infection can occur, such as a spot (pustule) or nodule.

Mild to moderate acne can usually be treated with antibacterial cream. In more severe cases, your GP may recommend antibiotic tablets.

Read more about acne.

Body odour

During puberty, your body develops large sweat glands around your armpits, breasts and genitals. These are known as apocrine glands.

Apocrine glands release sweat in response to stress, emotion and sexual excitement. In some cases, the excess sweat can cause body odour.

Periods

A girl's monthly periods usually start between 11-14 years of age (usually at 12-13). They continue until the menopause, which usually occurs around the age of 50.

In the days leading up to your period, you may have a number of symptoms, including:

• sore breasts
• irritability
• backache
• spots
• feeling very emotional or upset

These symptoms should pass once your period starts. Many girls and women have pain or cramping in their abdomen (tummy), back and vagina. This is often referred to as period pain. Taking paracetamol may help to relieve period pain.

Psychological and behavioural changes

Puberty can often be a particularly difficult time. You're forced to cope with changes in your body and possible side effects, such as acne or body odour, just at the time when you feel self-conscious about your body and self-image.

Puberty can also be an exciting time, as you develop new emotions and feelings. However, the 'emotional rollercoaster' experienced during puberty can have psychological and emotional effects such as:

• unexplained mood swings
• low self-esteem
• aggression
• depression 

These feelings can be a normal part of growing up and going through puberty. But if they're having a serious impact on your life, you may wish to talk to someone close to you, such as a friend or relative.

You could also ask your GP, or you could contact a support organisation, such as ChildLine, for free and confidential help and advice. Their helpline number is 0800 1111, or you could contact them through the ChildLine website.