Female Reproductive Physiology

At birth, women possess all the gametes that they will ever have. Thus, as women age, so do their gametes, and advanced oocyte age is associated with infertility. Many women, including women who are astronauts, delay childbearing until after they have completed their education and have achieved some of their career objectives. The average age of women selected into an astronaut class is 32 years, and many have not had children. Of 99 female finalist candidates examined during five selection cycles between 1989 and 1997, only 18 had been pregnant. Once they are admitted into the space program, given the constraints of training on a pregnancy, female astronauts commonly further delay childbearing until after the completion of one or two spaceflights. As a result, female astronauts are often in their 40s when they attempt pregnancy (Jennings and Baker, 2000).

Retrograde menstrual flow is considered an etiologic factor in the development of endometriosis and consequent infertility (Sampson, 1927; Scott et al., 1953; Jennings and Baker, 2000). Many women experience some retrograde menstruation that, at Earth’s gravity, is usually confined to the pelvis. However, in microgravity there is concern that the level of retrograde menstrual flow might be increased and that instead of being confined to the pelvis it would disperse throughout the abdominal cavity. Abdominal symptoms, shoulder pain, or an obvious reduction in the amount of menstrual flow has not been observed among women during spaceflights. However, retrograde menstrual flow has not been subjected to systematic study (Seddon et al., 1999).

Because of the short durations of space missions so far, coupled with the pulsatile nature of hormone secretion by the hypophyseal-pituitary-ovarian axis, the effect of space travel on ovulatory function has not been studied (Seddon et al., 1999; Strollo, 1999). The effects on the menstrual cycle of stress and exercise during space travel also have not been studied (Seddon et al., 1999). On Earth, stress and exercise can be associated with anovulation, and continuous estrogen exposure can be associated with endometrial hyperplasia and excessive vaginal bleeding. Alternatively, stress and excessive exercise can be associated with hypogonadotropic hypogonadism, resulting in reduced estrogen levels and amenorrhea. The latter condition is associated with decreased bone mineral density. The effect of microgravity, in addition to the effects of stress and an exercise program necessary to maintain cardiovascular and musculoskeletal well-being on long-duration flights, may increase the risk of developing one of these conditions. Exogenous



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