During labor and delivery, you might find yourself honed in on one aspect of making it to the finish line: hearing your baby take that first celebrated gasp of air. Indeed, your baby’s first breath marks a momentous feat. It means he’s ready to cut the cord — literally — and live outside of your womb. 

Until then, baby’s “breaths” in the womb look a lot different from your breaths and serve a wholly different purpose. Here’s how babies get the oxygen they need to survive in the womb, along with how they breathe during and after birth.

Do babies breathe in the womb?

Babies don’t “breathe” in the traditional sense in the womb — the air we inhale doesn’t exist in there!  

That said, babies do actually practice breathing, known as fetal breathing movements, well before they leave their comfy uterine confines. Muscle contractions bring amniotic fluid in and out of the lungs, which is thought to help strengthen the muscles involved in preparation for life outside of the womb. It may even support the development of alveoli, tiny air sacs on the lungs that exchange oxygen and carbon dioxide with blood. 

Although fetal breathing movements begin as early as week 10 of pregnancy, they start in earnest starting at about week 20. Since they’re not necessary for survival, practice breathing doesn’t happen constantly. In fact, babies may go a couple of hours without making any breathing movements at all. By weeks 24 to 28, practice breathing occurs about 10 to 20 percent of the time, increasing to 30 to 40 percent of the time after week 30.

Seeing these breathing movements on an ultrasound during the third trimester is just another sign that all is well.

How do babies breathe in the womb?

Although babies do practice breathing movements in the womb to train their muscles, they don’t actually breathe in oxygen and breathe out carbon dioxide like you do. Instead, oxygen and carbon dioxide exchange happen via the umbilical cord and placenta. 

The placenta seeps up oxygen molecules circulating in your blood and passes them along the umbilical cord to your baby. Baby’s waste products — including carbon dioxide — then pass back through the umbilical cord and placenta to your bloodstream for disposal. 

Outside of the womb, of course, things function very differently. The lungs distribute oxygen to the bloodstream when you breathe in and remove carbon dioxide from the bloodstream when you breathe out.  

How do babies breathe during and after birth?

During birth, babies continue to fill their oxygen needs through the umbilical cord and placenta. When your baby is delivered, his lungs are still filled with amniotic fluid thanks to all of that practice breathing in utero (which, by the way, stops when you go into labor). 

Within about 10 seconds of delivery, your baby will take his first breath (don’t be surprised when it sounds more like a gasp!) as he responds to the drastic change in environment. The fluid then begins to drain from his lungs and is replaced by oxygen, and the lungs begin to work all on their own just like yours. 

When are babies’ lungs fully developed?

By about week 28 of pregnancy, the lungs have developed enough that premature babies can breathe on their own — although the lungs and circulatory system still need some more time to mature. Around week 37, the lungs have developed more fully, and they'll keep on growing and fine-tuning until your little one is at least 8 years old. 

The lungs will start on one particularly important job at around month 6 of pregnancy: making surfactant, which allows them to inflate and deflate. This substance and alveoli (tiny air sacs at the end of airways) continue to develop right up until your due date. 

Artificial surfactant can help babies born extremely preterm (before 28 weeks), along with breathing assistance from a ventilator, CPAP machine or small breathing tubes placed in those teeny-tiny nostrils. Some babies born before 36 weeks and most babies born before week 32 may also need some breathing support. Luckily, these medical advances and a stay in neonatal intensive care unit (NICU) can treat respiratory problems and greatly help even the teeniest preemies. 

Wondering if there’s anything you can do to support your baby’s lung development now? Stick to the basics of a healthy pregnancy, including quitting smoking (if you currently light up) and following a nutritious diet. And once your little one is born, steer him away from smoke and encourage regular exercise to keep his lungs healthy.