How children’s lungs grow

This information will answer your questions about how your child’s lungs grow and develop. It explains how lungs develop in the womb and how they continue to mature after birth and through childhood

We have provided answers to questions such as:

You can read about how we breathe and how adult lungs work


How do a baby’s lungs develop in the womb?

Children’s lungs develop in 5 stages. These stages happen in the womb, but the final stage does not complete until later in childhood or early adolescence.

Do babies breathe in the womb?

Before they are born, babies’ lungs are filled with fluid. Your baby gets oxygen from the mother’s blood through the placenta.

The fluid in the womb lets your baby’s lungs develop and mature, ready for birth. They will not take their first breath of air until they are born.

Things always happen in the order we have outlined below, but the exact timings can be slightly different.

Stage 1: After conception

The first stage of your baby’s lung development happens at 3-5 weeks.

At 5 weeks your baby is just 2mm long, but the major organs are already beginning to form.

A lung bud develops from a tube of cells called the foregut (which will itself later go on to form the gut). This bud separates into two.

These two buds will eventually become your baby’s right and left lungs.

Your baby makes lung movements in the womb as if they are practising breathing. These movements start at the end of this stage.

Stage 2: Airways begin to form

The second stage of lung development happens from 5-16 weeks.

During this time your baby is growing rapidly. The major internal organs are in place by 12 weeks. At 14 weeks the baby measures 85mm from head to toe.

This is the stage where your baby’s lungs start to develop the tree-like structure you see in adult lungs.

Each lung bud starts to divide again and again, like the branches of a tree.

At first, they form 3 buds on the right side – these will become the upper, middle and lower lobes of the right lung. They only form 2 buds on the left side - the upper and lower lobes of the left lung. Your baby’s right lung will be bigger because the left lung has to share space with the baby’s heart. This is the same for almost everyone.

These buds continue to divide throughout this stage. They may divide up to 20 times.

By 16 weeks your baby’s lungs have all of their main airways (bronchi) and smaller airways (bronchioles). Cells that will eventually become the tiny air sacs (alveoli) have started to appear at the end of these smaller airways, like buds on trees.

Diagram showing how the lungs' air sacs develop in the womb

Stage 3: Getting ready to make air sacs and small blood vessels

Stage 3 takes place from 16-26 weeks.

Your baby starts to develop the areas where air sacs and blood vessels will eventually form, at the end of the smallest airways. These air sacs will be needed to get oxygen into their blood when they breathe outside the womb.

The cells that will become the air sacs carry on developing even after your baby is born.

Small blood vessels called capillaries grow close to these cells.

Stage 4: Preparing the lungs for breathing

This stage of development begins at 26 weeks and carries on until birth.

In this stage, the end of the smallest airways (now called saccules) grow in size. They will develop into early air sacs but they still don’t look like adult air sacs yet.

The walls of these growths get thinner to make more room for air in your baby’s lungs.

A substance called surfactant is produced during this stage.

Surfactant is a mixture of fats and proteins that help make sure the air sacs don’t collapse at the end of each breath out.

Stage 5: Air sac (alveoli) development

Stage 5 of lung development starts at 32 weeks and continues into childhood, after your baby is born.

In the last few weeks of pregnancy the first true air sacs (alveoli) develop.

More surfactant is produced as the lungs carry on developing.

The lungs develop and grow to enable oxygen to get into the blood. This prepares your baby’s lungs to breathe outside the womb.

Things always happen in this order but the exact timings can be slightly different.

In the womb the lungs are filled with fluid.  Your baby gets oxygen from the mother’s blood.


How do a baby’s lungs develop after birth?

Your baby’s lungs are ready to breathe as soon as they are born.

Estimates suggest a baby has around 20-50 million air sacs at birth. The number of air sacs in your child’s lungs will increase rapidly over the first two years of life.

New air sacs will keep developing at a slower rate until your child is at least 8.

There is now evidence that new air sacs keep developing until the teenage years. When they grow up, your child will have around 300 million air sacs.

The airways and lungs expand in size as your child grows taller, filling their rib cage.

Your baby’s first breath

When a foetus (unborn baby) is in the womb, their lungs are filled with fluid. As soon as a baby is born, they need to start using their lungs to breathe and get oxygen from the air.

Hormones and the pressure generated by the newborn’s loud cries help the lungs to remove the liquid that was in your baby’s lungs.

As an infant takes their first breath, the blood vessels around the lungs get wider and your baby’s blood flows to the lungs to collect oxygen.

Babies and toddlers: lung development from 0-3 years

Adult lungs have around 300 million air sacs. At birth your baby has 20-50 million air sacs. But they are still not fully developed.

In the first 6 months of your child’s life, the number of air sacs in their lungs rises sharply.

After 6 months, the number of air sacs continues to increase, but more slowly.

The amount of air your child’s lungs can hold is called lung volume.

Your child’s lung volume increases a lot in the first 2 years of life. This is because the number of air sacs in their lungs is growing so fast.

By the time your child is 3, their lungs look like a mini version of adult lungs.

Diagram showing what children's lungs look like at the age of 3

Your child’s maturing lungs: lung development from 3-8 years

Your child’s lungs get bigger as they grow.  All their airways and air sacs increase in size and volume.

The exact size and volume of the lungs will vary from child to child.

New air sacs keep developing throughout childhood and into the teenage years.


Promoting healthy lungs: what can parents do?

Childhood is a really important time for healthy lung development.

There is clear evidence that early exposure to air pollution can damage the lungs, and that it can increase the risk of lung infections that may be fatal.

Passive smoking (breathing in tobacco smoke when someone else has been smoking) can harm the lungs as they develop and slow down their growth.

The good news is that there are lots of ways you can keep your child’s lungs healthy. The most important things you can do to prevent lung damage are:

  • avoid smoking during pregnancy
  • make sure your child does not breathe in smoke once they’re born
  • avoid busy roads and junctions, and being stuck in traffic where air pollution can quickly build up
  • walk instead of driving, and choose less polluted, quieter routes if you can
  • take regular exercise to help lungs grow and keep clean. Make it fun: playing outside with friends is excellent for your child’s lungs and their physical, social, emotional and mental development.

Further information

Read more about how we breathe and how adult lungs work

Find out about risks to children’s lungs and what you can do to reduce them.

If you have concerns or need advice, call our helpline on 03000 030 555 between 9am and 5pm on a weekday or email them.

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