Because the diaphragm must function perfectly so immediately after birth, it is unlikely that a neonate's first breath is the first time the diaphragm has responded to nerve signals by contracting and expanding. Indeed, it has been recognized for over 60 years that the diaphragm engages in "practice" breathing-like patterns from early in gestation. Barcroft and Barron observed in 1936 that as early as G46 the diaphragm of fetal lambs contracts rhythmically, completing motions similar to those of breathing air. This suggests that the neural circuitry necessary for such patterned activity is functional and connected to the correct neural pathways by G46. The true respiratory pattern generator is not functional until G65, however. Cooke and Berger (1996) studied the development of patterns of activity in the diaphragm of fetal lambs in utero, from gestation age 45 days to 65 days (term=147 days). Diaphragmatic movements before G65 had not previously been investigated fully.
Early in the gestation of mammals, the respiratory pattern generating system develops and becomes functional, allowing the newborn to breath effectively as soon as it exits the womb. Using electromyogram measurements of the costal diaphragm from G45 to G65, Cooke and Berger detected both sustained motor activity as well as repetitive bursts of activity. Figure 13 shows the evolving pattern of the repetitive bursting activity, which Cooke and Berger suggest is evidence that this activity is produced by the developing respiratory generator in the brain. Additional support for this hypothesis is that this activity did not occur in a spinally transected fetus, the bursting always occurred on the right and left at the same time, and that by G65 the bursting had evolved into the stereotyped form known to be the precursor to adult respiratory motor pattern. The sustained activity, on the other hand, can be produced by the spinal cord alone, since spinally transected rat and chick embryos, as well as anencephalic human fetuses, have shown similar patterns of diaphragmatic activity.