When Are Babies' Lungs Fully Developed?

baby
Getty/Ariel Skelley 

One of the most important things that happens during pregnancy is the development of the lungs in a baby. 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 for 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.

Humans need lungs to breathe air, right? So, you could say the lungs are pretty important for a baby's growth and development. But when exactly are the lungs fully developed?

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 26-36 weeks, and then, finally, the last stage of lung development doesn't even begin until 36 weeks. That last stage occurs during the last month of pregnancy and even though it might seem like the baby is "done" 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, so that's why it is so important to do everything possible to let babies develop and choose their own birth dates, unless 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 for a baby if he or she is born too soon. The lungs are unique in that they are one the only systems in the body that stay primarily dormant until birth. Every other system, such as the cardiac system or the muscular system, are fully up and running even while the baby is still in-utero. But a baby in the womb actually gets his 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 the baby leaves 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 does involve so many factors going 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 on the lungs. 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 can be not 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, low levels of surfactant leads to a condition called respiratory distress syndrome (RDS) in babies, 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 Are a Baby's Lungs Fully Developed?

The Annual Review of Physiology journal explains an interesting fact: babies' lungs, although fully functional, are actually still technically not considered "fully" developed even at full-term birth. 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 three 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 three years of life, which increases the amount of surface area on the lungs. More alveoli = more air exchanged.

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. Most times, a doctor will weigh the need for a test with the risk of the baby being born early or the severity of the complications. If the baby is under 32 weeks, the test will most likely not be as helpful, since the lungs aren't likely to be developed enough for the test to pick up on. The test involves checking the amniotic fluid in the womb to measure the levels of surfactant. The doctors are able to determine how mature the lungs are by how much of the surfactant they can find in the fluid.

If it's found that a baby's lungs are not fully developed, a doctor may try to help the lungs along by ordering steroids that 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 adequate functioning lungs and babies have traditionally been considered "full-term" with normally-developed lungs by 37 weeks. However, 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.

What Affects Babies' Lung Development?

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

There are also factors that no parent can control that can affect lung development, such as the sex of the fetus and ethnicity. For instance, lung problems are more common in male babies as compared to female babies, and among black and South Asian infants more than any other race.

A Word From Verywell

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 because lung development can vary greatly.

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 unnecessary 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 he or she is 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 three years of life until they resemble the mature structure of an adult.

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