Final answer:
No, breathing does not occur during the canalicular development period; the fetus receives oxygen and expels carbon dioxide through the placenta. Fetal breathing movements begin around week 20, but they do not involve gas exchange with the environment as actual breathing does.
Step-by-step explanation:
Is breathing happening during the Canalicular development period? The answer to this question is, essentially, no. Breathing, as typically understood, involves gas exchange of oxygen and carbon dioxide, which does not occur in a fetus during the canalicular development period. Instead, the fetus receives oxygenated blood and the removal of carbon dioxide via the placenta provided by the mother.
From weeks 16 to 24, called the Canalicular stage, the developing respiratory system undergoes major developments including the formation of respiratory bronchioles, vascularization, and alveolar ducts. During this time, the fetal lungs fill with amniotic fluid, and while the fetus begins to make fetal breathing movements around week 20, these do not constitute true breathing. Instead, the movements prepare the respiratory muscles for breathing after birth and indicate fetal health.
By around week 28 in the later stage of development, termed the Alveolar period, the fetal lungs have developed enough for a prematurely born baby to potentially breathe on its own, given the maturation of alveolar precursors and production of surfactant adequate for lung compliance. However, full pulmonary function is not acquired until after birth, when actual breathing begins.