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their specialised functions in the nervous system. Therefore, they remain vulnerable to injury throughout childhood. Glands and hormones The fetus is able to produce hormones from early gestation. These are important for maintenance of pregnancy, and the fetus responds to stress by producing stress hormones. This can result in long-term resetting of hormone systems so that they do not function normally after birth. Because hormones produced by the endocrine system are vital for many functions of the body, including growth and development, such resetting can have adverse consequences for future health. Immunity, infections and allergies Babies' immune systems are not fully developed at birth and they are vulnerable to infections. Although the fetus is capable of producing antibodies and can develop allergic responses, most of the antibodies present at birth come from the mother, so the baby benefits from the mother's own immunity to infections. After birth, the immune system matures through exposure to infections and micro-organisms in the gut. In view of the very intricate and rapid changes in this early period of development, the fetus is very vulnerable to external insults, be they chemical, physical or types of radiation. Small changes or disturbances in the process can have permanent, lifelong consequences (Fig 7). A good example of this can be demonstrated by German measles (rubella) infection of a pregnant mother. The effect on the Fig 6. Principal stages of lung development in humans. Adapted with permission from: Kajekar R. Environmental factors and developmental outcomes in the lung. Pharmacol Ther 2007;114:129–45. Lung bud differentiation Trachea and bronchi Pulmonary vein and artery 4–7 weeks In utero Birth Postnatal 7–17 weeks 17–26 weeks 27–36 weeks 36 weeks – 2 years ~18 years Conducting airways Terminal bronchioles Immature neural networks Pre-acinar blood vessels Primitive alveoli Type II, type II cells Surfactant synthesis Alveoli saccules Extracellular matrix Neural network maturation Expansion of gas exchange area, nerves and capillaries Continued cellular proliferation Lung growth and expansion Terminal bronchiole Acinus Respiratory bronchiole Embryonic Pseudoglandular Canalicular Saccular Alveolar 3 In the beginning: protecting our future generations © Royal College of Physicians 2016 37

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