How children’s lungs grow

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

Diagram showing how the lungs develop in the womb

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.

Next: how do your baby's lungs develop after birth?

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Last medically reviewed: September 2019. Due for review: September 2022

This information uses the best available medical evidence and was produced with the support of people living with lung conditions. Find out how we produce our information. If you’d like to see our references get in touch.