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.
In the womb, babies don’t breathe air but practice “fetal breathing movements.” This involves drawing amniotic fluid in and out of their lungs to build muscle and support lung development, while oxygen and carbon dioxide exchange occurs through the placenta and umbilical cord. Babies rely on the umbilical cord for oxygen until delivery; within seconds of birth, they take their first breath, fluid drains from the lungs, and oxygen replaces it as the lungs begin working independently. Your baby’s lungs are functional enough for survival around 28 weeks, fully developed by about 37 weeks, and keep maturing through childhood. Surfactant production (key for inflation/deflation) begins around 6 months of pregnancy, with medical support available for preterm infants.
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.[1] It may even support the development of alveoli, tiny air sacs on the lungs that exchange oxygen and carbon dioxide with blood.[2]
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% of the time, increasing to 30 to 40% 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. “I had an ultrasound today (38 weeks, 2 days) to measure fluids and we caught him practicing breathing!” says What to Expect Community user dommysmommy. “I was not expecting it and it was amazing to see his chest and diaphragm moving up and down.”
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.[3]
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.[4]
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.[5]
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.[6] 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.[7]
"If a baby is in danger of being born prematurely, especially before 34 weeks of gestation, your doctor may give you steroids," says Jennifer Wu, M.D., a board-certified OB/GYN with her own private practice, Women's Health of Manhattan, in New York City, and member of the What to Expect Medical Review Board. "These are given as two injections 24 hours apart. The steroids help the baby’s lungs to develop faster and decrease the baby’s need for ventilation and supplemental oxygen after birth. Steroids can really help babies with extreme prematurity and improve their rates of survival."
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.
Interested in learning more about your baby’s development in the womb? Open the What to Expect app to track your little one’s growth from week to week, learn what to expect during labor and the newborn days, and much more.
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