Baby’s Lung Development At Second Trimester

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5 min read

Apr 16, 2025

Before birth, your baby cannot breathe. Right? Although he’s getting oxygen from the mother’s bloodstream through the placenta, inside, he is getting ready for his very first breath.

For this, your baby’s lungs are going through some major changes that you might not notice before the second trimester. It’s because weeks 13 to 26 is the time when the respiratory system starts developing.

But how does the lung develop in the second trimester, and what actually happens inside the mother’s body? Let’s have a detailed look below.

Early Lung Growth (Weeks 13–16)

In the initial weeks, 13-16, you’ll not notice any lung development. Instead, there will only be a network of small tubes or a small pouch of tissue called a lung bud. Then, at 6 weeks of pregnancy, a lower respiratory tract will begin to develop.

Over the next few weeks, the lung bud will divide into two separate buds and a windpipe will also form between them. At the end of 6 weeks, the lung buds will create a system of tubes called bronchi. In the initial stage, two bronchi will form and attach to each lung. Then, smaller branches will form within a few weeks.

Growing and Expanding (Weeks 17–20)

Once the bronchi are formed, the next development phase begins. At about 17 weeks, tiny air sacs called alveoli will start to form. These small tubes will help in holding oxygen and sending it to your baby’s bloodstream.

Although your baby is still getting respiratory help in the amniotic fluid, the breathing movement will start to develop. By the end of week 20, your baby’s lungs are stronger, complex but still not fully developed.

Major Development in Weeks 21–26

Here comes the most crucial development phase. At around week 24, a crucial coating named surfactants will start to appear. This coating keeps the air sacs open and prevents them from collapsing.

Meanwhile, the lungs continue their development, and by the end of week 26, they are already strong enough. Still, they are immature and need help breathing, but the chances of survival are much higher than just a few weeks ago.

What Happens If a Baby is Born Early?

Sometimes, there’s premature delivery, which means the baby is born before 26 weeks, so the lungs are not ready for development yet. Here are three different possibilities in this case:

Lungs need breathing support using a special breathing machine.
Doctors can give babies surfactant to help with breathing.
Preemies often spend time in the Neonatal Intensive Care Unit (NICU), where they get round-the-clock care until their lungs are strong enough to work on their own.

Final Thoughts

Conclusively, lung development is complete at 36 weeks. At this stage, the lungs are fully formed, and by the end of 40 weeks, your baby has almost 150 million alveoli in their lungs, which means your baby is ready to breathe. A reflex response is initiated when your baby takes his first breath. It’s almost 10 seconds after delivery.

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