As expectant mothers become more knowledgeable about their pregnancies, they are often confused about their babies and their ability to live in a uterus filled with fluid. You may ask yourself, “How does my baby breathe in the womb?” The short answer is, “She doesn’t.” Babies can’t actually breathe in the womb, but that doesn’t mean they don’t get oxygen; they just get oxygen in a different way.
In the womb, the fetal lungs start to develop around the third week of gestation. By 16 weeks of gestation, the general framework of the lungs is in place: two lungs with multiple airways in each lung. From 16 to 24 weeks gestation, the airways continue to lengthen, the capillaries in the lungs rapidly expand, and the lungs develop the ability to exchange oxygen and carbon dioxide. 1,2
But the lungs are not fully developed yet, which is why babies born at 24 weeks can survive (their lungs are able to perform the basic function of oxygen and carbon dioxide exchange), but need help from a ventilator to do so.
From 24 to 38 weeks gestation, the lungs finish their development by forming alveoli or breathing sacs in the lungs and produce surfactant, a chemical necessary to keep the infant’s lungs open and working.1,2
The womb and lungs, of course, are filled with amniotic fluid throughout pregnancy. This fluid is necessary for the lungs to expand and develop normally, and when amniotic fluid is low, the lungs don’t grow as well.
Late in pregnancy, the baby will exhibit fetal breathing movements, which look exactly as the name describes. These movements are important for lung development, as they help keep the lungs expanded and promote lung growth.2
Although these movements are vital, they don’t actually accomplish any oxygen exchange or breathing. So how does the fetus get oxygen if the lungs can’t actually breathe?
The Cord of Life
The umbilical cord contains two arteries and a vein that are connected to the placenta. The mother’s blood supply is also connected to the placenta. During pregnancy, when a mother breathes, her oxygenated blood goes to the placenta, down the umbilical vein and into the baby. The baby’s heart, like any heart, will pump the oxygenated blood out to body. The blood that comes back to the heart is now depleted of oxygen and has carbon dioxide in it from all the cells in the fetal body. This blood goes out the two umbilical arteries to the placenta, then out to the mother, where it travels to her lungs. There, the carbon dioxide is exchanged for oxygen, and the cycle repeats.
So in essence, the fetus doesn’t breathe in the womb, the mother breathes for her. Just as a mother is “eating for two,” she is also breathing for two.
A Sometimes Difficult Transition
But this all changes in the transition from the womb to extra-uterine life. During labor, the baby’s lungs change from secreting fluid to absorbing fluid. During delivery, traveling down the tight birth canal serves the purpose of squeezing a lot of the fluid out of the baby’s lungs. With the first few breaths, more fluid is absorbed and the lungs continue to clear fluid, a process that continues for the first few hours after birth.1,3
But with a Cesarean section, the baby does not get the benefit of this squeezing action, and may have a lot of retained fluid in the lungs. This causes a problem known as transient tachypnea of the newborn (TTN) in which the baby may breathe a little faster and work a little harder to breathe for the first day or so after delivery. Usually, the baby will resolve this problem on her own, but sometimes may need a little extra oxygen until the process is complete.
Another issue that comes up sometimes is a nuchal cord, when the umbilical cord is wrapped around the baby’s neck. Sometimes the nuchal cord may be compressed during a contraction, and the oxygen supply to the baby is then temporarily disrupted. This is usually not a serious problem, unless the cord is very tight.
So babies don’t actually “breathe” in the womb, but they do get plenty of oxygen from the mother via the umbilical vein. So do your baby a favor: relax, take a deep breath, and remember that you are breathing for two.
1 Morton SU, Brodsky D. Fetal Physiology and the Transition to Extrauterine Life. Clin Perinatol. 2016 Sep;43 (3):395-407.
2 Davis RP, Mychaliska GB. Neonatal pulmonary physiology. Semin Pediatr Surg. 2013 Nov;22 (4):179-84.
3 Hooper SB, te Pas AB, Kitchen MJ. Respiratory transition in the newborn: a three-phase process. Arch Dis Child Fetal Neonatal Ed. 2016 May;101 (3):F266-71.