Pituitary Hormone Analogs and Their Effects
During gestation, placental tissue secretes a number of substances that are physiologically similar to maternal pituitary hormones. This broad functional group includes human chorionic gonadotropin (hCG), placental lactogen (PL), adrenocorticotropic hormone (ACTH), and chorionic somatomammotropin, which is a direct functional analog of growth hormone.
In the maternal body, these biologically active substances effectively complement pituitary function. The critical role of hCG and placental lactogen is to support the corpus luteum: they continuously stimulate its growth and secretory activity in early pregnancy until the placenta is fully formed and capable of independently producing steroid hormones. Additionally, on the surface of chorionic villi, these hormones provide vital immunosuppression, suppressing the maternal immune response and preventing the immunological rejection of the embryo.
In the developing fetus, human chorionic gonadotropin acts as a potent stimulator: it triggers the production of its own ACTH and corticosteroids directly within the cells of the fetal adrenal cortex.
Female Sex Hormones (Steroids)
Normally, in a non-pregnant woman, sex steroid synthesis occurs in the ovaries. However, during pregnancy, the ovaries become functionally inactive because new follicles do not develop (preventing estrogen formation), and the corpus luteum eventually undergoes natural regression, completely ceasing progesterone production.
The placenta assumes this massive compensatory function. It begins to secrete required amounts of estrogens and progesterone, maintaining an optimal hormonal background. For the fetus, the presence of estrogens is absolutely critical, as they possess a pronounced mitogenic effect—strongly stimulating cell division and ensuring continuous growth of all embryonic tissues.
Relaxin and Preparation for Labor
In the later stages of pregnancy, the synthesis of a specific hormone—relaxin—acquires special functional significance. The main physiological goal of this substance is the comprehensive preparation of the female body for safe and natural childbirth.
Under the systemic influence of relaxin, large-scale structural changes occur in the tissues of the birth canal: the cervix significantly softens and dilates. The hormone also induces relaxation of the pubic symphysis and the pelvic ligamentous apparatus, which critically facilitates fetal passage. Simultaneously, relaxin effectively inhibits premature contractions of the smooth musculature of the uterus (myometrium), reliably preventing the early onset of labor.