Ground-based models have not been used frequently to study the effects of hypogravity on female reproduction. Women exposed to 17 days of 6° head-down tilt bed rest showed no changes in menstrual cycle length (Table 2). However, the duration of the study was too short to draw definitive conclusions. Rock and Fortney reported that women exposed to bed rest exhibited luteal phase deficiency, a condition that is related to HPG axis dysfunction. However, this study lacked controls. Clearly, more studies are needed to answer basic questions about female reproductive processes and to address concerns that fluid shifts associated with hypogravity may lead to retrograde menstruation or endometriosis.
To our knowledge, no researchers have used HLS to investigate hypogravity effects on reproductive processes in nonpregnant female rats, although HLS is ideally suited to answer pertinent questions. There is no concern about the translocation of reproductive organs in female rats because the ovaries are normally situated within the abdominal area. In addition, monitoring of the ovulatory cycles of HLS female rats is a simple technique that can provide important information on ovulation, reproductive capability, HPG, and ovarian function in nonpregnant females. A few HLS studies have been conducted on pregnant rats. HLS of rats during early pregnancy reduces implantation. The absence of elevated plasma or adrenal corticosterone in HLS pregnant rats suggests reproductive effects were due to simulated hypogravity exposure rather than stress. Similar findings derived from the spaceflight experiments raise questions as to whether early pregnancy can be sustained in hypogravity.