Fetal development: Your baby's lungs

Your baby's developing lungs

During pregnancy, your baby gets oxygen from your bloodstream through the placenta. So although he doesn't need lungs until he's born and takes his first breath, his respiratory system has been developing all along in preparation.

The process starts at 6 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 esophagus. Over the next week, this single bud will split into two separate buds (one for each lung), and the windpipe will start to form between them. 

At 8 weeks, the lung buds start branching internally to create the system of tubes (bronchi) that connect with the windpipe in a respiratory “tree." At first, two bronchi form and connect to each lung. As the weeks go by, smaller branches form off these main bronchi, and the branches get smaller and smaller as more are added.

At about 18 weeks, the smallest tubes (bronchioles) start to develop at the tips of the branches. At the end of these tiny tubes, respiratory sacs that eventually form the alveoli begin to appear.

By the time your baby is born, these sacs will become enmeshed with tiny blood vessels. This allows oxygen and carbon dioxide to flow in and out of the bloodstream in the process known as "gas exchange." This allows oxygenated blood to circulate to all of the body's organs and tissues through the arteries, while carbon dioxide is ferried back to the lung through the veins. 

From 24 weeks of pregnancy on (until your child is about 8 years old) these respiratory sacs grow and multiply, adding more surface area for the exchange of oxygen and carbon dioxide. Between 26 and 29 weeks, they start making a substance called surfactant, which coats the alveoli and keeps them inflated when there's not a lot of air in the lungs (when exhaling, for example). 

Even though your baby won't breathe on his own until birth, he starts practicing around 26 weeks by inhaling and exhaling small amounts of amniotic fluid. This process is essential for lung development and helps your baby prepare for life outside the uterus. 

By 28 weeks of pregnancy, gas exchange in your baby's tiny lungs may be possible, although the alveoli are not yet fully formed. There's also still not a lot of surfactant to support the baby breathing on his own. If your baby is born now, he'll need a lot of intensive care, including surfactant therapy. He may also face lifelong respiratory problems as a result of being born very early.

By 35 weeks of pregnancy, there may be enough surfactant to support lung function, though he'd still need some help breathing. He may need to spend time in a specialized hospital facility for preterm babies known as the neonatal intensive care unit (NICU).

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.

So how does your baby know to take his first breath? It's most likely a reflex response to touch and being exposed to air for the first time. (Contrary to what you see on TV, slapping a baby's buttocks isn't necessary, though some babies may need a gentle rub with a towel.)

When your baby inhales for the first time, his lungs expand, and any fluid remaining in the alveoli is replaced with air. The alveoli then start the life-sustaining process of gas exchange.

What you can do during pregnancy

Preterm birth is the greatest threat to the development of your baby's lungs, so call your healthcare provider right away if you notice any of the following signs of preterm labor:

• More vaginal discharge than usual
• A change in the type of discharge – like if you're leaking watery fluid or if your discharge becomes watery, mucusy, or bloody (even if it's pink or just tinged with blood)
• Any vaginal bleeding or spotting
• Abdominal pain that feels like menstrual cramps, or having more than four contractions in one hour (even if they don't hurt)
• Increased pressure in your pelvic area (a feeling that your baby is pushing down)
• Low back pain, especially if it's dull or rhythmic, or you didn't previously have back pain

These symptoms can be confusing because some (such as pelvic pressure or low back pain) are common during pregnancy, and sporadic early contractions may be Braxton Hicks contractions.

But it's always best to be cautious, so call your provider right away if you have any unusual symptoms at any time during your pregnancy.

Key milestones in fetal lung development

Learn more:

Slideshow: Fetal development, week by week