Sudden infant death syndrome (SIDS): microgravity and inadequate sensory stimulation.
In early gestation, the human foetus develops in a buoyant environment, which is similar to the near-weightlessness of space flight. After the 26th week of gestation, the foetus gradually becomes exposed to gravitational forces. The influence of fluid immersion declines as the weight of the foetus increases. In this way, the foetus adapts and trains for the gravity environment after birth. Failure of gravitational loading in the last trimester of pregnancy delays development and maintains the pathophysiological environment of microgravity as experienced by the astronaut in space flight. The deconditioning effects of microgravity during space flight are the reverse processes of intrauterine development after the 26th week when the foetus begins training body processes for adaptation to an earthly environment. Growth requires space and movement, which suggests that a growth-retarded foetus may have been deprived of the mechanical dimension of uterine wall pressure, and, in twins, the smaller sibling may have been deprived of space. The behaviour of a study group of sudden infant death syndrome infants suggested a continuation of the effects of the foetal akinesia syndrome during the third trimester period of gestation. NASA research into the pathophysiology of microgravity was based on a simple insight: that the physiological effects of human space travel were virtually identical to the adjustments the body makes when lying down. This is the same environment as that of the human foetus in the first 22 weeks of gestation after which the uterine environment becomes a prelude to adaptations to the force of gravity.