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La biología del desarrollo prenatal




THE BIOLOGY OF PRENATAL DEVELOPMENT

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Capítulo 41   4 a 5 meses (16 a 20 semanas): respuesta al estrés, vérnix caseoso, ritmos circadianos

By 16 weeks, procedures involving the insertion of a needle into the abdomen of the fetus trigger a hormonal stress response releasing noradrenaline, or norepinephrine, into the bloodstream.

In the respiratory system, the bronchial tree is now nearly complete.

A protective white substance, called vernix caseosa, now covers the fetus. Vernix protects the skin from the irritating effects of amniotic fluid.

From 19 weeks fetal movement, breathing activity, and heart rate begin to follow daily cycles called circadian rhythms.

Capítulo 42   6 a 7 meses (24 a 28 semanas): reflejo de parpadeo; las pupilas responden a la luz; olfato y gusto

By 20 weeks the cochlea, which is the organ of hearing, has reached adult size within the fully developed inner ear. From now on, the fetus will respond to a growing range of sounds.

Hair begins to grow on the scalp.

All skin layers and structures are present, including hair follicles and glands.

By 21 to 22 weeks after fertilization, the lungs gain some ability to breathe air. This is considered the age of viability because survival outside the womb becomes possible for some fetuses.