From Embryology
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Maternal pregnancy is divided into three stages of growth and development - the first trimester which involves organogenesis via cellular proliferation, differentiation, and morphogenesis; the second trimester which is centred around organ development and functioning as well as foetal expansion and recognisable movement (quickening); and the third trimester during which the foetus undergoes rapid weight gain.

Second Trimester Foetal Development

The second trimester encompasses the foetal weeks 12-24, or clinical gestational weeks 14-26, according to the UCSF Medical Centre and is referred to as the "golden period of pregnancy" since many first trimester discomforts such as nausea, fatigue, breast tenderness, and frequent urination are alleviated as the maternal body adapts to the changing gestational physiology - nevertheless increased placental secretions of progesterone, estrogen, and human placental lactogen, as well as increased abdominal pressure can elicit heartburn, constipation, abdominal pain, and leg cramps. During the second trimester of pregnancy, the organ rudiments formed in the embryo stage expand in size and further differentiate in response to cellular signalling and gene cascades to coordinate specialised physiological functions needed to support the growing foetus. In addition to foetal growth and development, the placenta also increases in size to increase foetal blood supply and secretes hormones to redirect maternal glucose, amino acids, and free fatty acids towards the foetus to satisfy its increasing metabolic demands.

Skin and Musculoskeletal System

During the second trimester

Respiratory System

Early lung branching and morphogenesis. Source
Development of lungs and respiratory tree-like system.

The primary function of the respiratory system is to facilitate gaseous exchange between the external environment and the bloodstream, through diffusion across the lung alveoli, in order to oxygenate pulmonary blood via inhalation and remove carbon dioxide via exhalation. In the embryonic and foetal stages of development, the lung is collapsed and its function is replaced by the placental circulation which coordinates the oxygenation of blood and the diffusion of carbon dioxide into the maternal bloodstream. The respiratory system recevies only 7% of foetal blood supply in utero.

Differentiation of the lateral plate mesoderm gives rise to the mesenchyme of the lung; including its connective tissue, the smooth muscle surrounding the airway and blood vessels, endothelial cells which form vasculature, the cartilage, lympathic drainage systems, and mesothelial cells lining the external pleural surface. Early lung morphogenesis in the second trimester is controlled by various cell signalling proteins such as fibroblast growth factor, sonic hedgehog protein, gives rise to a tree-like system of epithelial tubules and sacs surrounded by vasculature.

  • In the embryonic stage, the endodermal foregut forms a ventral evagination called the respiratory diverticulum which bifurcates into the right and left primary bronchial buds; these structures differentiate into the main bronchi and the respiratory diverticulum forms the trachea and larynx.
  • In the second trimester, the lungs are in the pseudoglandular (weeks 6-16) and undergo expansive branching into bronchopulmonary segments with terminal bronchioles.
  • In the canalicular stage (weeks 16-28) of respiratory development, the terminal bronchioles further divide into multiple respiratory bronchioles which form terminal sacs (primitive alveoli) in the early third trimester.
  • Vascular endothelial growth factor and angiogenic growth factor result in the development of respiratory vasculature - a dense network of blood vessels, especially capillaries, form through the recruitment of endothelial cells in close apposition with the lung epithelium. This process continues during the third trimester as the mesenchyme surrounding the alveoli becomes vascularised to facilitate rapid gaseous exchange by diffusion.
  • Maturation of lung epithelium initiates in the second trimester through epithelial-to-mesenchymal interactions in the lateral plate mesoderm, resulting in the rudiments of ciliated, secretory, and neuroendocrine epithelium in the proximal bronchioles and Type 1 (squamous supporting cells) and Type 2 (secrete pulmonary surfactant) in the distal alveoli.

Since alveoli development occurs in the early third trimester, pre-term babies born in the second trimester have low concentrations of pulmonary surfactant which is needed to lower the surface tension of alveolar fluid and keep the alveoli patent (open) during inhalation. This can cause pulmonary edema (alveoli filled with proteinaceous fluid) which leads to Respiratory Distress Syndrome as greater intra-thoracic pressure needs to be generated for inhalation. Corticosteroids (cortisol) can be administered to stimulate production of pulonary surfactant.

Central Nervous System

Gastrointestinal System

Urogenital System