Lung development begins in the early stages of pregnancy, around the fourth week, when the embryonic lungs start to form. This process involves the growth of lung buds that eventually branch out into the complex structures needed for breathing. By the end of the first trimester, the basic architecture of the lungs is established. As pregnancy progresses, the lungs continue to mature, with significant changes occurring in the second and third trimesters. By about 24 weeks, the lungs produce surfactant, a substance that helps keep the air sacs open, allowing for efficient gas exchange at birth. Full lung function is typically achieved after birth as the baby takes its first breaths.