Hypergravity effects on pregnancy and parturition.

The events of mammalian parturition (labor and birth) were analyzed in Norway rats exposed to either 1.0-g, 1.5-g or 2.0-g hypergravity (hg) beginning at the approximate midpoint of their pregnancies. Pregnant dams were 24h timelapse videorecorded during parturition (days 22-23). Compared with 1-g controls, hg dams displayed one-half as many lordosis contractions, the predominant labor contraction type in rats. The timing of birth, duration of delivery and number of neonates born was comparable in hg and 1-g control dams. Results are discussed in relation to earlier findings that, following spaceflight during pregnancy, lordosis contractions are doubled in frequency. The importance of labor in the transition from prenatal to postnatal life is discussed. Together, these results provide evidence that gravitational load varies inversely with lordosis contraction frequency and support the view that certain biological systems show a continuum of response across gravitational vectors.