An outstanding role during pregnancy is the creation in the body of a new endocrine balance. From the moment of nidation of the fetal egg into the wall of the uterus and the formation of the trophoblast, the synthesis of the last chorionic gonadotropin begins. Due to the luteotropic effect of HG in the ovaries with the menstrual yellow body, the yellow body of pregnancy is formed. It is able to synthesize progesterone and estrogenic hormones (mainly estrone and estradiol). The yellow body persists throughout gestation, but its maximum activity is observed at 4-5 weeks of gestation.
Beginning with the third month of the gestational process, degenerative changes are observed in the yellow body of pregnancy and from this time its function is assumed by the placenta. As the endocrine organ of the placenta operates under conditions of a single utero-placental-fetal system.
In the placenta (syncytium, cytotrophoblast and decidua), hormones are formed, both protein and steroidal. The level of estrogen hormones from the beginning of the period of active function of the placenta (from the 13th week of pregnancy) increases significantly and subsequently progressively increases throughout pregnancy. This is because against the background of increased synthesis of estrone and estradiol in the ovaries of the mother, the level of estriol, products of the feto-placental complex also grows. And 90% of it is formed in the liver of the fetus from dehydroepiandrosterone, produced by the adrenal glands of the fetus and only 10% - in the placenta from the mother's predecessors (estrone and estradiol). The constantly growing concentration of estrogens ensures the growth and development of the uterus, the regulation of biochemical processes in myometrium, the increase in the activity of enzymatic systems, the intensification of energy metabolism and the accumulation of glycogen and ATP necessary for fetal development.
Changes in the qualitative composition of estrogens occur during the period of preparation and development of labor: the fractions of active generators of inductors (estradiol and estriol) are increasing. No less important steroid hormone is progesterone, which prepares the body for pregnancy and ensures its normal course. This is due to the ability of this hormone to inhibit the contractile activity of the uterus, to maintain the tone of the isthmic-cervical department, and to stimulate the growth of the uterus and vessels during pregnancy. The level of progesterone increases especially after 12 weeks of pregnancy. By the end of pregnancy, it increases 10-20 times.
In addition to steroid, about 10 hormones of protein nature are synthesized in the placenta. Among them, the most important are chorionic gonadotropin and placental lactogen.
Chorionic gonadotropin is produced by the cytotrophoblast throughout the entire pregnancy, but the maximum of its production falls on the first 10 weeks. It is at this time that the yellow body of pregnancy actively functions, the maintenance of development of which is the main purpose of the chorionic gonadotropin.
From the 12th week and during the entire gestational period, the production of placental lactogen increases, which plays a role in the production of pulmonary surfactant and fetoplacental osmoregulation. Placental lactogen is a polypeptide hormone that has lacto-, somato- and luteotropic activity. Its main role is to regulate carbohydrate and lipid metabolism, enhance protein synthesis in the fetus. This hormone is produced by placenta syncytiotrophoblasts, starting from the 5th-6th week of pregnancy. 90% of the placental lactogen enters the blood of the pregnant woman, and 10% - into the amniotic fluid. During the gestational process, the concentration of placental lactogen gradually increases, reaching a maximum level at 36-37 weeks of gestation. After that, the level of the hormone stabilizes to 39 weeks, and then decreases. After birth, the placental lactogen quickly disappears from the mother's blood.
An important role in the physiological functioning of the mother-placenta-fetus system is played by prostaglandins (especially E2 and F2a). They themselves do not belong to real hormones, because they are not secreted by glands of internal secretion, but are synthesized by all tissues of the body. But these biologically active substances can enhance or weaken the action of hormones on target cells. By regulating the biosynthesis of cyclic adenosine monophosphate in them, endogenous prostaglandins affect almost all parts of the reproductive system, including the activity of spermatozoa in cervical mucus, ovulation processes and steroidogenesis in the ovaries, implantation of the egg, the course of pregnancy, induction and regulation of labor, lactation.
The endocrine activity of the ovaries and the feto-placental complex determines a new mode of operation of the central glands of internal secretion . Under the influence of constant impulses of the cerebral cortex, the activity of the hypothalamus increases, which leads to an enhanced synthesis of releasing factors of the hormones of the pituitary and neurohormones (vasopressin and oxytocin). Vasopressin is involved in the regulation of volumic-osmatic homeostasis, activates the function of the fetal hypothalamus and affects the secretion of corticotropin. The level of this hormone, to a certain extent, depends on estrogenic stimulation. In the first weeks of pregnancy, it is slightly reduced, and then significantly increases. Oxytocin increases the activity of myometrium and stimulates the cells of the mammary glands, which determines its importance in the induction of labor, the course of labor and the maintenance of normal lactation in the postpartum period. In addition to uterotonic actions, it has moderate vasopressor and antidiuretic activity and slightly increases the aggregation of blood elements. By the time of delivery, the concentration of oxytocin in the mother's blood reaches its maximum level. However, it was found that even in this period, the content of oxytocin in the feto-placental system is much higher than in the maternal system, which indicates the participation of the hypothalamic-pituitary fetal system in the preparation of the pregnant woman for delivery.
Noticeable histological and functional changes occur in the work of the adenohypophysis. In the first 3 months of pregnancy, the synthesis of luteinizing hormone and prolactin increases. The latter prepares mammary glands for lactation, and the constant increase in its level in the blood of a pregnant woman with an increase in the duration of the gestational process is explained by the stimulating effect of a large number of steroid hormones, especially estrogens.
At the same time, pituitary hormone production (FSH, LH, ACTH, TSH, STG) is increasing in pituitary production, it stimulates the peripheral endocrine glands by the principle of direct binding to the secretion of the corresponding hormones required in conditions of intensification of metabolism and fetal growth. In close connection with the level of ACTH, there is a cortex of adrenal glands, which leads to its hypertrophy during pregnancy and associated with it pronounced functional changes. The synthesis of glucocorticoids regulating carbohydrate and protein metabolism, as well as the metabolism of lipids, is greatly enhanced, with which hyperlipidemia and hypercholesterolemia of pregnant women are associated. The level of mineralocorticoids, before aldosterone, is constantly growing, which is increasing sodium reabsorption and potassium excretion in the harvesting tubes of the cortex of kidneys, plays an important role in the regulation of water-salt metabolism. Under the influence of a large number of sex hormones that weaken the sensitivity of the vessels to vasopressor substances, the function of the adrenal medulla is compensatory, which leads to an increase in the biosynthesis of catecholamines and an increase in the level of adrenaline and norepinephrine in the blood.
With the development of pregnancy, the function of the thyroid gland is activated, an intensified synthesis of its hormones occurs. The number of bound forms of thyroid hormones increases, and the level of free, active fractions of thyroxin and triiodothyronine corresponds to that of pregnancy. The main role of thyroid hormones is metabolic, and it consists in increasing the utilization of oxygen, ATP, activation of protein synthesis, which is extremely important for normal modulation of the growth and development of the fetus. As pregnancy progresses, the synthesis of thyrocalcitonin increases. This hormone is in a dynamic antagonistic balance with the parathyroid hormone, which causes a decrease in its activity during pregnancy. Therefore, the level of parathyroidin in the process of development of pregnancy falls, causing a violation of phosphorus-calcium metabolism, which explains the possibility of hypocalcemia and the court.
Certain changes are established in the pancreas. Beginning with the second trimester of pregnancy, the production of insulin increases, which is necessary to maintain the blood glucose level in pregnant women under conditions of distinct intensification of carbohydrate metabolism in connection with the ever growing energy needs of the fetus. Thus, the function of almost all the endocrine glands increases, creating a new endocrine balance to ensure a normal course of pregnancy.