Mammalian reproduction evolved within Earth’s 1-g gravitational field. Therefore, deviations from Earth’s normal gravity, i.e., hypogravity (forces < 1 g) or hypergravity (forces > 1 g), may compromise reproduction. Some interesting findings are emerging from spaceflight studies. For example, a transient but dramatic reduction in testosterone (T) has been reported during spaceflight in male rats and humans. This observation suggests that fertility may be reduced during spaceflight, because adequate levels of T are required by adult males to maintain reproductive function. Pregnancy during spaceflight has been contraindicated, leading female astronauts to suppress their fertility cycles.
As a result, no systematic studies have been conducted to investigate the effect of deviations from 1 g on the reproductive physiology of nonpregnant mammals. Several studies have reported no detrimental effects on pregnancy, reproductive hormones, fetal development, parturition, or lactation in female rats following return to Earth.
In the era of the International Space Station with the eventual goal of space colonization, the emphasis of space biology research has begun to shift from investigations of acute responses to studies of chronic effects of altered gravity. if space is to be successfully colonized, then a more comprehensive understanding of the effects of altered gravity on mammalian reproduction is needed. In addition, an understanding of the effects of altered gravity on reproductive physiology is imperative for the development of countermeasures. Here, we review the existing data on mammalian reproduction derived from spaceflight studies and from ground-based models of simulated hypo- and hypergravity. Such studies will provide the foundation to permit multiple generations of humans and other mammals to exist in space.