In which trimester does the following occur? the respiratory system becomes mature.

third trimester

During the third trimester, the airspaces of most peripheral airways are mostly widened and lengthened, fibroblastic cells produce extracellular matrix, collagen and elastin and the vascular tree increases in diameter and length. These developments support epithelial differentiation and surfactant secretion. At week 32, the baby practices his breathing.

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explanation:

it's a nutrition food for pregnant mothers so they unborn child can get its nutrition that it needs to grow in to a healthy baby.

explanation:

and the mother is getting all of her nutrition and her calcium and vitamins .