Gravity and gravidity : will microgravity assist pregnancy?

During the third trimester of pregnancy, exercise, deep ventilation and supine position may be particularly uncomfortable for pregnant women. Reduction of fetal weight by a microgravity environment may relieve some of this discomfort. This study assessed, by means of heart rate variability (HRV), the cardiovascular adaptation to the supine posture, to relaxing respiratory exercises (deep slow breathing) and to mild and moderate lower limb exercises in pregnant women, both in a dry environment and during a head-out water immersion, which mimics microgravity. Six third-trimester pregnant women (31±3 years; 67.8±8.3 kg; 163±3 cm; gestational age 26±6 weeks) were enrolled. Five-minute series of beat-to-beat RR intervals were recorded by a heart rate monitor in basal (B) state (on a separate day, comfortably sitting in a quiet room, 22° C) and in 2 different environments (G, gym, 22° C; W, water immersion at 34° C), on separate days, in 5 experimental conditions for each environment: (1) resting supine; (2) during upright mild lower limb exercise; (3) during upright moderate lower limb exercise; (4) during controlled deep ventilation (breathing rhythm, 0.1 Hz); (5) during controlled breathing (supine). Compared to B state, HR increased slightly in all experimental conditions in both G (+16%) and W (+8%) environments, whereas blood pressure did not. LF/HF ratio, a marker of sympathovagal balance, increased significantly during G at rest (5.0±2.4, p<0.05) but not in W at rest (2.7±1.5), and further increased during mild and moderate exercise in both G and W. During slow breathing in both environments (conditions 4 and 5), LF/HF paradoxically increased compared to B. Finally, LF/HF significantly increased (6.5±2.6, p=0.01) during ventilation in supine position, but only in G environment. Only HF decreased in both G_rest (-46.2%, p<0.05) and W_rest-24.6%, p<0.05) compared to B state, whereas LF was almost unchanged. Immersion in thermoneutral water, which mimics microgravity, produces some positive effects on the autonomic adaptation to different manoeuvres (supine position, exercise, deep slow ventilation) in third-trimester pregnant women. These effects are especially relevant in the assumption of supine posture, suggesting that microgravity will probably relieve the symptoms of aorto-caval compression produced by the fetus burden.