Babies do not breathe air while developing inside the womb. Instead, a fetus receives all the oxygen it needs directly from the mother. This process ensures the baby’s developing lungs remain filled with fluid, creating an environment essential for their growth and maturation until birth.
How Babies Get Oxygen
A fetus receives oxygen and nutrients through the placenta, which develops within the mother’s uterus during pregnancy. The placenta acts as an interface, facilitating the exchange of substances between the maternal and fetal bloodstreams. Oxygen from the mother’s blood diffuses across the placenta into the fetal circulation. The umbilical cord connects the fetus to the placenta, serving as the conduit for this exchange.
The umbilical vein within the cord carries oxygen-rich blood from the placenta to the fetus, while two umbilical arteries return deoxygenated blood and waste products back to the placenta. Inside the fetus, the circulatory system features temporary shunts that redirect blood away from developing organs, such as the lungs and liver. For instance, the foramen ovale allows oxygenated blood to bypass the lungs by flowing directly from the right atrium to the left atrium of the heart, and the ductus arteriosus shunts blood from the pulmonary artery to the aorta. This fetal circulation ensures that oxygenated blood reaches organs without passing through the fluid-filled lungs. Additionally, fetal hemoglobin has a higher affinity for oxygen compared to adult hemoglobin, helping the fetus efficiently absorb oxygen from the placenta.
Lung Development Before Birth
Even though they do not breathe air, fetal lungs undergo development throughout pregnancy, preparing them for independent respiration after birth. Lung development progresses through several stages: embryonic, pseudoglandular, canalicular, saccular, and alveolar. During the canalicular stage, between 17 and 26 weeks of gestation, the initial air-blood barrier begins to form, and surfactant production begins. Surfactant, a mixture of lipids and proteins, is important for reducing surface tension within the air sacs, preventing them from collapsing. Its production increases during the saccular stage, from 26 weeks until birth, preparing the lungs for gas exchange.
The fetal lungs are filled with fluid, which is essential for their growth and expansion. This lung fluid is produced by the fetal lungs and contains components supporting maturation. The fetus engages in “fetal breathing movements,” rhythmic contractions of the diaphragm and other respiratory muscles. These movements, observed as early as 15 weeks of gestation, help strengthen respiratory muscles and promote the development of airways and air sacs. While these movements cause amniotic fluid to move in and out of the lungs, they do not involve gas exchange.
The First Breath
The transition from a fluid-filled to an air-filled respiratory system at birth is a complex event. As a baby is born, several stimuli trigger the first breath, including the sudden change in temperature, tactile stimulation, and the clamping of the umbilical cord. The pressure changes during passage through the birth canal help to expel some fluid from the baby’s lungs. Remaining fluid is then either absorbed by the lymphatic system or actively transported out of the airspaces.
The newborn must generate negative pressure to overcome the surface tension within the collapsed air sacs and inflate the lungs. The presence of surfactant is important in this process, allowing the alveoli to expand and remain open. As the lungs inflate with air, the oxygen levels in the pulmonary blood vessels increase, causing them to relax and decrease resistance to blood flow. This increase in blood flow to the lungs, combined with the loss of the placental circulation, causes the temporary shunts in the fetal heart, such as the foramen ovale and ductus arteriosus, to close, rerouting blood to flow through the lungs for oxygenation.
Why Babies Don’t Breathe Air in the Womb
The fluid-filled state of the fetal lungs is a protective design, important for their healthy development. Breathing air while submerged in the amniotic fluid would be comparable to drowning and would cause severe damage to the lung tissues. The presence of fluid within the lungs maintains the pressure for the air sacs and airways to grow and mature. Without this fluid, the lungs would collapse, hindering their development.
The amniotic fluid also provides a sterile and protective environment. It cushions the fetus from external impacts and helps prevent infections, contributing to healthy development. The fetal respiratory system is adapted to function in a liquid medium, and its transition to air breathing is timed to occur at birth, when independent respiratory function is needed.