The fetal lung undergoes complex developmental processes to prepare for air breathing after birth. While in the womb, the lungs do not perform gas exchange, a function handled by the placenta. Instead, they focus on structural maturation and fluid management, laying the groundwork for independent respiration.
Stages of Fetal Lung Development
Fetal lung development is a sequential process divided into five overlapping stages, beginning early in gestation and continuing into childhood.
The embryonic stage (approximately day 26 to day 52, or weeks 3-7) initiates lung formation with the appearance of the respiratory diverticulum from the foregut endoderm. During this period, the trachea, major bronchi, and diaphragm begin to form, with initial airway branching. By the end of this stage, lung primordia, lobes, and bronchopulmonary segments are established.
The pseudoglandular stage spans from roughly day 52 to week 16 of gestation. In this phase, airways continue to branch, forming the extensive bronchial tree, which resembles a gland. The respiratory tree develops as far as the terminal bronchioles, accompanied by the formation of an arterial system, cartilage, and smooth muscle.
The canalicular stage occurs from approximately week 17 to week 26. This period is marked by the development of vascular beds and capillaries, along with the formation of respiratory bronchioles and alveolar ducts. The differentiation of the airway epithelium becomes visible, and the bronchioalveolar duct junction forms.
The saccular stage occurs from about week 26 to week 36. During this time, the airspaces expand, forming saccules, which are precursors to the alveoli. The gas-exchange surface area of the lungs significantly expands as these terminal structures develop.
The alveolar phase begins around week 36, extending to term and continuing into early childhood, potentially up to 8 years of age. This stage is characterized by the formation of true alveoli, the tiny air sacs where gas exchange will occur. At birth, a newborn typically has around 50 million alveoli, with further maturation and multiplication occurring postnatally.
Fetal Lung Function and Transition at Birth
The primary role of the fetal lung in utero is to produce and maintain fetal lung liquid. This liquid, secreted by the bronchopulmonary epithelium, helps maintain the lungs in a distended state, promoting their expansion and structural maturation.
Fetal lung liquid leaves the trachea and is either swallowed by the fetus or enters the amniotic sac, contributing to the amniotic fluid. The volume of this liquid is regulated by the resistance of the upper airway to efflux and by fetal breathing movements, which begin around 20-21 weeks of gestation. These movements involve muscle contractions that cause the inhalation and exhalation of amniotic fluid, helping to tone respiratory muscles in preparation for breathing after birth.
The transition from intrauterine to extrauterine life involves rapid physiological changes to enable independent breathing. A surge in fetal catecholamines and steroids, often triggered by labor, plays a significant role in preparing the lungs for this shift. A key adaptation is the clearance of fetal lung fluid. As the fetus is squeezed through the birth canal, the thoracic cavity is compressed, expelling a substantial amount of this fluid.
The remaining fluid is rapidly absorbed shortly after birth. Simultaneously, surfactant, a foamy substance, coats the tiny air sacs (alveoli), reducing surface tension and preventing them from collapsing. Surfactant production begins around 24 weeks of gestation and increases as term approaches, with sufficient amounts present by 34-36 weeks. The first inhalation, occurring within 10 seconds of birth, inflates the lungs, supported by surfactant.
Common Conditions Affecting Fetal Lung Health
Several medical conditions can impact fetal lung development and function, leading to respiratory challenges at birth.
Respiratory Distress Syndrome (RDS), also known as hyaline membrane disease, is common in premature infants born before 37 weeks gestation. This condition results from insufficient surfactant in the lungs, a substance necessary to keep the air sacs open. Without enough surfactant, the alveoli collapse with each breath, making it difficult for the baby to take in oxygen. Symptoms often appear within minutes to hours of birth and can include rapid breathing, grunting sounds, nasal flaring, and chest retractions.
Lung hypoplasia is a condition where the fetal lungs do not develop as expected, resulting in underdeveloped and often smaller lungs. It is almost always caused by an underlying health issue. Causes often include conditions that limit the space available for lung growth, such as low levels of amniotic fluid (oligohydramnios) or structural problems that restrict chest development. Severe cases can lead to low blood oxygen levels and high blood pressure in the lungs, known as pulmonary hypertension.
Congenital diaphragmatic hernia (CDH) is a birth defect where there is an opening in the diaphragm, the muscle separating the chest and abdominal cavities. This opening allows abdominal organs, such as the stomach and intestines, to move into the chest cavity, occupying space meant for the developing lungs. The presence of these organs restricts normal lung growth, leading to pulmonary hypoplasia and often pulmonary hypertension, where the blood vessels in the lungs are underdeveloped and resist blood flow. Babies with CDH almost always experience breathing problems after birth, which can be life-threatening.