Fetal Lung Development Stages And Phases: Overview And Study Guide

In comparison to the prenatal development of the other organs in the fetus, the development of the lungs requires special attention.

The activities of the lungs as the breathing organs are not considered essential for the intrauterine existence of the baby. However, their development must be normal and optimum such that they are ready to function immediately after birth.

This explains why the development of the lungs from the embryonic phase up to birth throughout the fetal period and even later needs to be understood clearly. The development of the lungs in the fetus during different stages of intrauterine life is discussed below.

The development of lungs in babies

Before a baby is born, his or her lungs are filled with fluid and the baby receives the supply of oxygen and other nutrients from the mother’s blood via the placenta.

It is the fluid in the womb that allows the baby’s lungs to develop and mature, such that they are ready to function independently at birth. However, the babies do not take their first own breath of air until they are born.

The development of lungs occurs in the order outlined below, though the exact timings may be slightly different.

Stage 1: Embryonic phase           

This is the first stage of the lung development that begins about 3 to 5 weeks after conception. At 5 weeks, the baby is only 2 mm long. However, the major organs begin to develop at this phase.

The embryonic phase of pulmonary development starts with the formation of a tiny groove called the sulcus laryngotrachealis in the ventral lower pharynx.

At this stage, a lung bud that develops from the tube of cells known as the foregut (that later forms the gut) separates into two. These 2 buds eventually become the right and left lungs of the baby.

The movement of the lungs begins at this stage as if they are practicing the process of breathing. These movements begin at the end of the embryonic stage.

Stage 2: Formation of Airways

The 2nd stage of lung development begins at 5 weeks and continues until 16 weeks of pregnancy.

During this phase, the baby is growing rapidly. The major organs of the body are in place by the 12th week. At 14th week, the baby measures around 85 to 90 mm from head to toe.

At this stage, the baby’s lungs begin to develop into a tree-like structure that characterizes the structure of the adult lungs. Each bud divides repeatedly to form branches like that of a tree.

Initially, 3 buds are formed on the right side. These buds become the 3 lobes of the right lung called the upper, middle, and lower lobes. Similarly, 2 buds are formed on the left side that grow and develop forming the upper long and lower lobe of the left lung. The right lung of the baby is bigger as the left lung has to share the space with the heart just like in adults.

The buds continue to divide up to 20 times throughout this phase.

By 16th week, all of their main airways including the bronchi and the smaller airways like the bronchioles are formed. The cells that eventually become the small air sacs called alveoli begin to appear at the ends of these smaller airways, just like the buds on trees.

Stage 3: Formation of air sacs

This phase occurs from the 16th to the 26th week of pregnancy. It is marked by the development of the areas where small air sacs and blood vessels eventually form, at the end of the small air passages. These air sacs play a key role in the exchange of gases between the blood and air passages after the child is born.

Small blood vessels grow closer to these cells to facilitate the exchange of gases.

Stage 4: Preparation of the lungs for breathing

This phase of development begins at the 26th week of pregnancy and continues until the birth.

During this stage, the ends of the smallest airways, which are now called saccules, begin to grow in size. They develop into early air sacs, though they do not look like the adult air sacs.

The walls of these sacs become thinner to make room for the air in the baby’s lungs.

A substance known as surfactant is produced at this stage. It is a mixture of proteins and fats that ensures the air sacs do not collapse at the end of each cycle of breathing when the lungs force the air out from the air passages.

Stage 5: Alveoli development

The 5th stage of lung development begins at the 32nd week of pregnancy and continues into the childhood, even after the baby is born.

During this stage, the true air sacs called alveoli develop, and more surfactant is produced. The lungs grow and develop to enable oxygen to enter the blood circulation. This prepares the baby’s lungs to breathe immediately after birth.

How do the lungs develop after birth?

The lungs of a baby are ready to breathe immediately after birth. A newborn baby has around 20 to 50 million alveoli at birth. The number of air sacs in the child’s lungs continues to increase rapidly over the first 2 years of life.

New alveoli keep developing until the child is at least 8 years old. The lungs and airways also expand in size as the child grows taller, thus filling their rib cage.


The development of lungs in a baby is the most critical aspect of neonatal development as it determines the ability of the child to sustain existence following birth. You can learn more about the development of the lungs in the fetus by attending our respiratory conference. You can attend the AARC approved CEU’s to keep yourself updated about the latest research studies in the field of pulmonary medicine so that you can provide better treatments to your pediatric patients diagnosed with conditions linked to the abnormal lung development.