How Babies' Lungs Develop In and Out of the Womb

mother holding newborn baby at hospital

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One of the most important things that happens during pregnancy and fetal growth is the baby's lung development. Fully-developed lungs are one of the key factors necessary for life outside of the womb. Many of the other parts of a baby are functioning quite early on during fetal development, but for the lungs, every day of development is important.

Even one day can make a difference in lung development. For babies who are at risk of being born prematurely, for example, doctors focus primarily on making sure that the lungs are as developed as possible before the baby is born, so the baby has the best chance of survival.

How Babies' Lungs Develop

Lung development in humans happens over five different stages. After the embryo stage, a baby's lungs develop in what's called the pseudoglandular stage. During this stage, which lasts from about 5 weeks to 17 weeks gestation, the baby's lungs can be compared to a tree trunk with branches sprouting from it. As the baby grows, the "branches" become more involved and complex.

The next stages occur in phases, from 26 weeks to 35 weeks, and then, finally, the last stage of lung development, which doesn't even begin until 36 weeks. That last stage occurs during the last month of pregnancy. And though it might seem like the baby is done developing by then, there is actually a tremendous amount of growth that happens in that last stage of lung development.

During that last month, the baby's lungs do the majority of developing that they need to function outside of the womb, which is why it is so important to do everything possible to let babies develop and choose their own birth dates unless it becomes medically necessary to deliver early.

The lungs are actually one of the last things to finish developing in a baby, which is why an under-developed set of lungs can be so dangerous if a baby is born too soon.

The lungs are unique in that they are one of the only systems in the body that stay primarily dormant until birth. Every other system, such as the cardiac system or the muscular system, is fully up and running even while the baby is still in-utero. But a baby in the womb actually gets their oxygen supply from the placenta, so the lungs don't get their official "test run" until the moment of birth.

A baby does some "practice" breathing in the womb, but there isn't any actual air exchange within the lungs until after they leave the womb. The entire process of lung development is very complex and involves many different functions, so when it's time for them to spring into action, it's a crucial moment.

Unfortunately, because it requires so many factors to go right, there are a lot of opportunities for things to go wrong, too. When a baby is born and once the umbilical cord is clamped, it has to switch over from "breathing" through the placenta's blood supply to breathing actual air.

During that moment, the baby's lungs expand with air, the "flap" on the heart shuts to start the circulation from the lungs and the new system of getting oxygen into the blood from the air kicks in. Sometimes, that process can take some time, and, especially if the baby is born prematurely, there can be problems getting enough oxygen into the body.

Lung Development at Birth

The most important part of a baby's lung development is something called surfactant. Surfactant is a mixture of primarily fatty acid components, carbohydrates, and proteins that "coat" the lungs and allows them to work properly. It helps to keep the alveoli, which are the air sacs where all the oxygen exchange happens, open and inflated.

The surfactant is what develops last, and there is a risk that it will not be fully present if a baby is born too early. When there isn't enough surfactant in the lungs, the baby is not able to breathe properly.

Most often, a low level of surfactant leads to a condition called respiratory distress syndrome (RDS), especially premature babies. The baby tries very hard to breathe, but the lungs just aren't able to work properly to get the air exchange needed.

In premature babies, RDS is the number one cause of death.

When Do Babies' Lungs Fully Develop?

The Annual Review of Physiology journal explained an interesting fact back in 1984: Babies' lungs, although fully functional, are actually still technically not considered "fully" developed even at full-term.

Remember those five stages of lung development? Well, you may be surprised to hear that the very last stage of lung development continues from 36 weeks gestation all the way through a child's first few years of life. 

During the first 3 years of a child's life, the lungs continue to develop and mature into the structure of an adult lung. More specifically, alveoli (the small "sacs" that exchange air in the lungs) continue to form over those first 3 years of life, which increases the amount of surface area on the lungs. More alveoli equal more air exchanged.

Testing Babies' Lung Function

There is no official way to know if the lungs are developed before the baby is born without doing invasive testing. In some cases, such as if there are complications with the pregnancy and doctors need to deliver the baby early, or if the mother is at extreme risk for preterm delivery, they might order tests to determine the baby's lung function.

However, as of 2019, the American College of Obstetricians and Gynecologists (ACOG) says that testing amniotic fluid for lung maturity isn't of much clinical value. If a physician is worried about pre-term birth, they may try to help the baby's lungs along by ordering steroids, which are injected into the mother while she is still pregnant. These drugs can help "speed up" the process of lung development.

In general, most babies born at 35 weeks will have adequately functioning lungs and babies have traditionally been considered "full-term" with normally-developed lungs by 37 weeks.

The American College of Obstetricians and Gynecologists now recommends that babies should not be induced or delivered before 39 weeks of pregnancy unless testing to make sure the lungs are fully developed has been done.

Babies can develop at different times and the bottom line is, a baby's lungs are always developing, so every day counts during pregnancy.

Many factors affect how a baby's lungs develop in the womb. Smoking, for example, has been found to damage the fetal lungs even before the pregnancy is over. This means that smoke and nicotine specifically can cross the blood-placenta barrier.

Rate of Lung Development

Although it varies, a baby's lungs are not considered fully-functioning until around 37 weeks gestation, which is considered "full-term." However, because conception and development can happen at different rates, this not a hard and fast number.

Some babies born earlier might have fully-functioning lungs, and some babies born later might still have issues with their lungs at birth. Lung development is one of the most crucial components of a baby's growth and it's one of the primary reasons why doctors encourage mothers to avoid elective inductions that aren't for a medical reason.

If preterm delivery is inevitable, a doctor might order special medication to help the baby's lungs function better. Medication and support interventions might also help a baby after they are born if there are problems with the lungs.

A baby's lungs are considered fully-functioning at full-term birth, but a child's lungs will also continue to develop in the first 3 years of life until they resemble the mature structure of an adult.

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Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Chaunie Brusie, RN, BSN
Chaunie Brusie is a registered nurse with experience in long-term, critical care, and obstetrical and pediatric nursing.