How do babies breathe in the womb?

As your pregnancy continues, your baby will practise breathing movements, though, to get themselves body ready for when they're born and will need to breathe for themselves.

How does a baby breathe in the womb?

Well, they don't. Breathing is the act of inhaling air into the lungs and then expelling, or breathing it out, again. Babies can't do that in the womb, as they are surrounded by amniotic fluid and not air.

But babies do use oxygen and do produce carbon dioxide (as a waste product of respiration). So, how?

It's all about the umbilical cord– which is the connection between the placenta and your baby. The umbilical cord carries the oxygen your developing baby needs from you to your baby.

Essentially, you breathe in oxygen and it's absorbed, as molecules, into your blood; the oxygen molecules are carried in your blood around your body including to the placenta; the oxygen molecules that circulate in the placenta then pass along the umbilical cord to your baby's bloodstream.

As well as you delivering oxygen into your baby, you're also taking waste products, such as carbon dioxide, out – using the same umbilical-cord pathway but in reverse. This means that, as you breathe out, you're also breathing out carbon dioxide from your baby.

Amazingly, during this whole process of moving substances between your baby and you, your blood supply and your baby's blood supply don't mix. And the umbilical cord has separate blood vessels, so that the vessels supplying oxygen to your baby and the vessels taking carbon dioxide away from your baby don't mix.

How does a baby breathe during labour?

Well, they still don’t! Throughout labour, they continue to use the oxygen delivered to them through the umbilical cord.

This is why having your baby in a birth pool will not affect your baby's supply of oxygen. Once your baby is delivered, though, it must be brought out of the water.

Sometimes, during labour or delivery, the umbilical cord can become wrapped around your baby's neck. This usually doesn't prevent it providing your baby with oxygen – unless it's wrapped very tightly round. Midwives are trained to check the position of the cord – and, if possible unwrap it if there is this kind of issue.

How do a baby's lungs develop in the womb? When are they fully developed?

Your baby's lungs start to develop from about week 6 of your pregnancy, and they continue to develop throughout the whole of the rest of your pregnancy.

The lungs begin as a tiny lung bud that then divides into two buds, each of which will go on to form a separate lung with the connecting stem between them developing into your baby's windpipe.

Over the following weeks, each lung will develop a branch-like system of smaller air tubes (bronchi) inside it. Then at about 18 weeks, even smaller tubes (bronchioles) start to branch off the bronchi and, from 26 weeks, tiny air sacs called alveoli start to develop at the end of each bronchiole.

Although, as we've seen above, your baby isn't using their lungs to breathe in oxygen while they're in the womb, they will be spending increasing amounts of time practising the art of breathing. They'll start this in small stints as early as 10 to 12 weeks of pregnancy and, by 30 weeks, they're spending up to 40 per cent of their waking time taking practice breaths – inhaling and exhaling small amounts of amniotic fluid. (Remember, this is OK as they are getting their oxygen from you.)

The lungs are still continuing to develop right up almost to the end of pregnancy – the alveoli multiply and get thinner and acquire blood vessels, all increasing the surface area that can exchange oxygen and carbon dioxide.

So, if your baby is delivered prematurely, they may have breathing difficulties because some aspects of the lung haven't fully developed yet. For this reason, if you go into premature labour, you may be offered steroid injections to help your baby's lungs mature before delivery. Other support such as oxygen, CPAP (continuous positive airway pressure) and ventilation may be needed once they have been born.

Even after delivery, your baby's lungs aren't fully developed: lung development actually continues for the first few years of childhood.

How does a baby start to breathe when they're born?

The switchover after delivery from your baby receiving oxygen from your through the umbilical cord to your baby breathing on their own is extremely complex. But actually starts to happen within about 10 seconds of birth!

When your baby is born, all the alveoli (air sacs) in your baby's lungs are filled with amniotic fluid – thanks to all that practice breathing.

The change in the temperature after delivery – from the warm internal environment of the womb to the colder air outside – triggers your baby to take their first in-breath. With the first breath, this fluid is pushed out as the lungs expand, the pressure in the lungs decreases and blood flow to the lungs begins. There are also structural changes going on in your baby's heart, closing the openings which existed in the heart before birth and directing blood flow from the heart to your baby’s lungs.

Generally, this first breath is a reflex and is spontaneous, though it can be helped along by stimulation – drying your baby and rubbing them lightly. Some babies will need a little more help than others.

Pic: Getty Images

Read more:

• What is placenta?
• How often should I feel my baby move - week by week?
• When will I first feel my baby move?