Fetal development: your baby's lungs

Graphic of fetal lung development
Jonathan Dimes for BabyCenter

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

During pregnancy, your baby gets oxygen from your bloodstream through the placenta. Although he doesn't need lungs until he's born and takes his first breath, his respiratory system starts to develop from very early on.

The process starts at six weeks of pregnancy, when a small pouch of tissue called the lung bud is created from the front wall of the tube that will become your baby's food pipe (oesophagus) (Hill 2020, Moore et al 2019). Over the next week, this single bud will split into two separate buds, one major air tube (bronchus) for each lung, with the stem between them forming your baby’s windpipe (trachea) (Hill 2020, Moore et al 2019)

At eight weeks of pregnancy, the lung buds start branching to create a system of air tubes (bronchi) that connect with your baby’s windpipe. The branches form a kind of respiratory "tree" in each lung (Moore et al 2019). As your baby grows and develops, more branches sprout off these main bronchi, getting smaller and smaller as more are added (Hill 2020, Moore et al 2019). From as early as ten weeks of pregnancy, your baby is making breathing movements in your womb.

By about 18 weeks of pregnancy, the smallest air tubes (bronchioles) are developing at the tips of the branches and these start to fill out (Moore et al 2019). At around 26 weeks of pregnancy, tiny respiratory sacs develop at the end of these bronchioles. These will continue to multiply and develop, becoming the alveoli.

The alveoli have a vital function in your baby’s lungs; allowing oxygenated blood to circulate to all his organs and tissues through his arteries, while ferrying carbon dioxide back to the lung through his veins.

From as early as 21 weeks, cells within the respiratory sacs start making a substance called surfactant (Cau et al 2016, Martin 2019, Moore et al 2019). Surfactant coats the developing alveoli. Later on, it will keep the lungs from collapsing when there's not a lot of air in them, for example, when exhaling (Hill 2020, Tommy's 2017). Some babies have more surfactant than others (Cau et al 2016), but most have enough in their lungs by about 26 weeks to 28 weeks of pregnancy. This will help them to survive if born prematurely, though they may need to be given tiny quantities of extra surfactant in hospital to help them take in oxygen (Moore et al 2019, Tommy’s 2017).  

At 24 weeks to 28 weeks of pregnancy, your baby spends up to 20 per cent of his time inhaling and exhaling small amounts of amniotic fluid (Plosa and Guttentag 2018). This process is essential for lung development and helps your baby prepare for life outside the womb (Moore et al 2019). By 30 weeks of pregnancy, he’s spending up to 40 per cent of his time practicing his breathing (Plosa and Guttentag 2018).

At 34 weeks of pregnancy, your baby’s respiratory sacs have grown and multiplied. Their walls have become much thinner and they are now known as alveoli (Moore et al 2019). Tiny blood vessels begin to bulge into the sacs. These help to create more surface area to exchange oxygen and carbon dioxide. This process is known as "gas exchange" and it can start from as early as 28 weeks of pregnancy, before the alveoli are even fully formed (Martin 2019, Moore et al 2019).

By 40 weeks, your baby has about 150 million alveoli in his lungs, ready to take over the job of breathing once he's born (Martin 2019, Moore et al 2019). By the time your child is eight years old, he will have another 150 million, which is the same number as you (Moore et al 2019).

So how does your baby know to take his first breath? It's most likely a reflex response to the sudden change in temperature, touch and being exposed to air for the first time (Hillman et al 2012, Ohning 2019). Contrary to what you see on TV, slapping a baby's bottom isn't necessary, though some babies may need a gentle rub with a towel to encourage them to take their first breath.

When your baby inhales for the first time, his lungs expand, and any fluid remaining in the alveoli is replaced with air (Moore et al 2019). The alveoli then start the life-sustaining process of gas exchange.
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What are the milestones in my baby’s lung development during pregnancy?

Weeks pregnantWhat's happening
6 weeksLung bud develops.
7 weeksLung bud branches into two buds, one for each main air tube (bronchus).
8 weeks More air tubes (bronchi) start to form in the lungs.
10 weeksBreathing movements begin.
18-26 weeks Bronchi branch into smaller air tubes (bronchioles) which start to expand.
26 weeks onwardsRespiratory sacs develop at the end of each outer bronchiole. They continue to grow, multiply and become coated in protective surfactant.
24-30 weeks Your baby spends around a third of his time inhaling and exhaling amniotic fluid (practice breathing).
28 weeksGas exchange may be possible.
35 weeksThere's usually enough surfactant coating the maturing respiratory sacs (alveoli) to support lung function at birth.
40 weeks There are now 150 million alveoli in the lungs, and another 150 million will form during childhood.

How can I help my baby’s lungs develop during pregnancy?

Premature birth is the greatest threat to the development of your baby's lungs, so call your midwife or maternity unit right away if you notice your waters breaking, regular contractions or bleeding before 37 weeks (NHS 2019).

Sometimes, the signs that labour has started may be subtle, such as dull cramps or backache. Find out some of the other signs of premature labour.

It's always best to be cautious, so call your midwife or maternity unit right away if you have any unusual symptoms at any time during your pregnancy.

Learn more:

Last reviewed: January 2020


Cau F, Pisu E, Gerosa C, et al. 2016. Interindividual variability in the expression of surfactant protein A and B in the human lung during development. Eur J Histochem 60(3):2678. www.ncbi.nlm.nih.gov[Accessed January 2020]

Hill MA. 2020. Respiratory system development. University of New South Wales Embryology. embryology.med.unsw.edu.au [Accessed January 2020]

Hillman N, Kallapur SG, Jobe A. 2012. Physiology of transition from intrauterine to extrauterine life. Clin Perinatol 39(4): 769-83. www.ncbi.nlm.nih.gov  [Accessed January 2020]

Martin R. 2019. Pathophysiology, clinical manifestations, and diagnosis of respiratory distress syndrome in the newborn. UpToDate. www.uptodate.com [Accessed January 2020]

Moore KL, Persaud TVN, Torchia MG. 2019. Respiratory system. Chapter 10 in The developing human: clinically oriented embryology. 11th edition. Elsevier

NHS. 2019. Premature labour and birth. NHS, Health A-Z, Pregnancy and baby. www.nhs.uk [Accessed January 2020]

Ohning BL. 2019. Neonatal resuscitation. Medscape. emedicine.medscape.com [Accessed January 2020]

Plosa E, Guttentag SH. 2018. Cyclic stretch: fetal breathing movements. In: Gleason CA, Juul SE, eds. Avery's diseases of the newborn. 10th ed.

Tommy's. 2017. Breathing support for premature babies. Tommy's, Your baby's time in hospital. www.tommys.org [Accessed January 2020]

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