Developing babies need oxygen beginning early in pregnancy. But a baby won't take their first breath until after birth. This means that babies don't truly breathe in the womb. Instead, the umbilical cord provides the baby with oxygen until the first breath.

Lung development begins early in pregnancy, but is not complete until the third trimester. Between 24–36 weeks of pregnancy, the lungs begin developing alveoli - the tiny lung sacs that fill with oxygen. Until these sacs are fully developed, a baby may have difficulty breathing on its own outside of the womb.

Women giving birth sometimes worry about how their babies will breathe, especially as the baby travels down the narrow confines of the birth canal. The umbilical cord continues to supply a baby with oxygen until after it is born.

Fast facts on how babies breathe in the womb:

  • In the earliest weeks of pregnancy, a developing baby looks more like a ball of cells than a person. In these early weeks, there's no need to breathe.
  • The umbilical cord is the main source of oxygen for the fetus.
  • As long as the umbilical cord remains intact, there should be no risk of drowning in or outside the womb.

Several biological systems and processes play a role. They include:

The umbilical cord

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The umbilical cord provides a baby with oxygen-rich blood in the womb.

After 5-6 weeks of pregnancy, the umbilical cord develops to deliver oxygen directly to the developing fetus's body. The umbilical cord connects to the placenta, which is connected to the uterus. Both structures house many blood vessels, and continue to grow and develop throughout pregnancy.

Together, the umbilical cord and placenta deliver nutrients from the mother to the baby. They also provide the baby with the oxygen-rich blood necessary for growth.

This means that the mother breathes in for the baby, and the oxygen in her blood is then transferred to the baby's blood. The mother also breathes out for the baby, as carbon dioxide from the baby is moved out through the placenta to the mother's blood, the removed with exhale.

Substances going into the developing baby, such as oxygen, never interact with the substances leaving the baby, such as waste products. They travel through the umbilical cord through two separate blood vessels.

Lung development in the womb

Lung development is normally complete after 35-36 weeks of pregnancy. However, development varies and it's possible to miscalculate when a baby was conceived. This is why even late preterm babies often experience difficulties breathing. Steroids may help speed the development of a baby's lungs. When a woman must give birth early, or when she is at risk of preterm labor, doctors may recommend steroids given to the mother to improve a baby's chances of survival outside the womb.

Even when a fetus's lungs are fully developed, it's impossible for the fetus to breathe until after birth. Developing babies are surrounded by amniotic fluid, and their lungs are filled with this fluid. By 10–12 weeks of gestation, developing babies begin taking "practice" breaths. But these breaths provide them with no oxygen, and only refill the lungs with more amniotic fluid. Because it's normal for a fetus's lungs to be filled with fluid, a fetus can't drown in the womb.

If there is a problem with the placenta or umbilical cord, there's no other way for a developing baby to breathe. As a result, issues with these structures can cause birth defects, brain injuries, or even the death of the fetus.

Some babies are born with the umbilical cord wrapped around the neck. This relatively common issue, called a nuchal cord, happens in 12-37 percent of births. In most cases, it causes no problems. This is because the umbilical cord is still able to provide the baby with oxygen.

However, if the cord is wrapped very tightly around the baby's neck, the oxygen supply in the cord might be limited. During birth, the care provider will check for a nuchal cord, and if possible, unwrap the cord. Once the baby is born, the new environment - which includes temperature changes, a lack of amniotic fluid, and exposure to air - triggers the baby's first breath.

Some babies have their first bowel movement during birth, before exiting the womb. This stool is called meconium. During a practice breath during or shortly before birth, a baby may inhale meconium. Inhaling meconium can be serious and can harm a baby's ability to breathe outside the womb. So babies who have inhaled meconium may need treatment with suction and oxygen after birth.

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A water birth won't affect a babies ability to breathe.

Many hospitals are offering water birth, which some women prefer to traditional birth options. Women giving birth at home or in birthing centers may also choose water birth. Water birth can be soothing, may help with pain relief, and mimics the environment of the womb. It is generally safe, and won't affect a baby's ability to breathe.

This is because the baby will continue getting oxygen from the umbilical cord until removed from the birthing tub. A baby left in the birthing tub for too long could theoretically drown. Isolated reports suggest that it's possible for a baby to be injured during water birth. However, a 2009 Cochrane review that looked at 12 previous studies of water birth found no increase in the risk of harm to the baby. Upon delivery the baby is brought up and out of the water and takes its first breathe then.

It is worth noting that both the American College of Obstetricians and Gynecologists and the American Academy of Pediatrics do not recommend water delivery.

When a baby does not get enough oxygen during and immediately following labor and birth, it is called hypoxia. Hypoxia deprives the brain and body of the oxygen they need to properly function. This can cause a range of birth injuries, including cerebral palsy and death. Common causes of hypoxia include:

  • Cord problems, such as a damaged cord, or a cord with damaged blood vessels.
  • Abnormal presentation. Some babies born breech suffer from oxygen deprivation at birth.
  • Shoulder dystocia, which occurs when the shoulders get stuck, slowing delivery after the head has emerged.
  • Excessive bleeding during pregnancy or birth.

Quality prenatal care and an attentive care provider during birth can greatly reduce the risk of hypoxia. A baby experiencing hypoxia may need supportive care, such as oxygen therapy or a ventilator.