Once conception occurs, numerous endocrine changes take place on an almost-daily basis. The bidirectional communication between the mother and the fetal-placental unit relies on a host of hormonal factors. The hormones with associations to sleep will be described. Progesterone is the most described and understood hormone during pregnancy and is thought necessary for the maintenance of the pregnancy. Progesterone is a steroid hormone that is produced primarily by the placenta and is synthesized by maternal cholesterol (Liu 2004). It has a variety of roles, including uterine and mammary gland development and the onset of labor (Astle, Slater, and Thornton 2003). It is also the primary contributor to whether uterine quiescence is present throughout gestation (Buster et al. 2003). Progesterone is known to have immunosuppressive properties and is thought to be an important component of successful pregnancy when high levels are present at the maternal-fetal interface; very high levels of progesterone can induce production of Th2-type cytokines such as IL-4 and IL-5 (Piccinni, Scaletti, Maggi, and Romagnani 2000). Maternal serum levels increase as pregnancy progresses and secretion sites of progesterone increase so that by the end of the third trimester most of the progesterone is secreted from the fetal-placenta unit (Buster et al. 2003; Liu 2004; Porterfield 2001; Taylor, Lebovic, and Martin-Cadieux 2001).
Estrogens, a group of steroid hormones, have various roles throughout a person's life, but during pregnancy their primary function is to regulate progesterone and maintain maturation. The pregnancy-related estrogen, estriol, is a hormone originating almost exclusively from the placenta thus its production in nonpregnant women is minimal (Buster et al. 2003). It plays a significant role in augmenting uterine blood flow and in the timing of labor stimulation (Buster et al. 2003). It increases as the pregnancy moves toward term (Porterfield 2001) and is a fundamental marker of the progression to spontaneous labor (Goodwin 1999). A steep increase in estriol output that is accompanied by a reduction in progesterone is observed at approximately 3 weeks before parturition. For women who had abnormal delivery times, i.e., past their due dates, salivary estriol levels and the ratio of estriol:progesterone were lower than for women who delivered spontaneously (Goodwin 1999). In comparing preterm and term labors, the surge of estriol has been documented to occur about 4 weeks sooner in those who experience preterm labor (McGregor et al. 1995). Furthermore, low maternal concentrations have been associated with fetal and placental development problems including fetal abnormalities and congenital derangements (Buster et al. 2003; Taylor et al. 2001).
There are a few other hormonal factors that change during pregnancy and are involved in the sleep process. During pregnancy, corticotropin-releasing hormone (CRH), which is normally synthesized by the hypothalamus, is synthesized by the placenta, fetal membranes and decidua and is physiologically identical to that of the mother (Florio, Cobellis, Woodman, Severi, Linton, and Petraglia 2002; Wadhwa, Sandman, Chicz-DeMet, and Porto 1997). CRH is a crucial element in parturition and fetal growth and varying levels have been shown to be involved with preterm labor (Inder et al. 2001; Siler-Khodr, Forthman, Khodr, Matyszczyk, Khodr, and Khodr 2003; Wadhwa et al. 1997). Prolactin is a hormone produced by the anterior pituitary gland with its primary role being initiation and sustenance of lactation. The secretion of prolactin is increased by stress and is dependent upon a woman's estrogen levels. Finally, cortisol, a well-known steroid hormone, is particularly involved in the pregnancy process. The levels of maternal cortisol are associated with the rise in estrogen production. However, the rate of secretion of cortisol by maternal adrenals is not increased in pregnancy; rather, only the rate of clearance is decreased (Goodwin 1999). Cortisol is critical for maturation of the lungs, liver, and other tissues of the developing fetus (Goodwin 1999).
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