Review of experimental studies on the functional development of the respiratory system in the foetal lamb.

In the foetus, as in the adult, the behavioural state of the central nervous system plays a very significant role in the modulation of respiratory activity. Breathing at birth represents the continuation of a process that commences early in life in utero and develops as gestation proceeds. The profile of classical afferent respiratory input to the foetus in utero is significantly different to that of the new born, with a reduction in total neural traffic and a smaller phasic variation. The foetus, however, phasically contracts its diaphragm which, as gestation proceeds, is patterned into distinct 'on and off' periods. Breathing of the foetus occurs during rapid eye movement sleep, analogous to that in the adult, and continues following somatic stimulation. This behavioural state is reduced during foetal hypoxia and foetal respiration is simultaneously depressed. At birth there is an increase in both somatosensory neural input and phasic variation and the new born breathes in all behavioural states, the stability of which varies cyclically with a period similar to that seen before birth. Whereas improved surgical and experimental techniques and non-invasive technology have enabled a more complete description of the development of the respiratory system in utero, the transitional events of the processes which occur at birth happen very quickly and at present prevent a similar detailed description.