Pregnancy is a period punctuated by intense and obvious changes in the female organism, supported in large part by hormonal changes that begin even before conception. Let us recall briefly how from the time of ovulation - which occurs roughly in the middle of each menstrual cycle - the levels of progesterone begin to increase in order to prepare the uterus for pregnancy.
Main functions of progesterone :
- contributes to the development of the product of conception before it is implanted, specifically enhancing the tubal and uterine secretions necessary for the nourishment and development of morula and blastocysts (aggregates of cells that form in the first stages of embryogenesis after fertilization);
- prepares a uterine environment suitable for a possible installation;
- induces the development of decidual cells in the endometrium, important for the nutrition of the embryo in the early stages;
- inhibits the contractility of the gravidar uterus, avoiding the possibility of a miscarriage;
- prepares the mammary gland for lactation, favoring tubulo-alveolar development.
A few days after conception, the levels of human chorionic gonadotropin, a hormone produced by the trophoblast and the placenta that derives from it, also begin to increase. Human chorionic gonadotropin keeps the corpus luteum formed following ovulation active; in this way the levels of progesterone and estrogen continue to increase, also thanks to the increasing contribution of the placenta. Thus, starting from the third month of pregnancy, the levels of human chorionic gonadotropin tend to decrease quite clearly, stabilizing around the twentieth week.
Estrogens and progesterone constantly increase during pregnancy to preserve the endometrium, prepare the mammary gland for lactation and suppress the development of new ovarian follicles. In the first trimester of pregnancy, the origin of the stereoide hormones is mainly represented by the corpus luteum, thanks to the support of the human chorionic gonodotropin; in the second and third quarters, it is instead the placenta that fulfills this function.
Main functions of human chorionic gonadotropin (HCG).
It stimulates the corpus luteum to secrete even higher amounts of its hormones, such as estrogen and especially progesterone, avoiding regression. These hormones, as anticipated, ensure that the endometrium (innermost lining of the uterus) continues to develop and store large quantities of nutritive material, protecting it from the flaking experienced by every woman at the time of menstruation.
Thanks to the growth stimulus and hormonal secretion of the corpus luteum, human chorionic gonadotropin maintains the decidual characteristics of the endometrium, necessary for the first stages of development of the placenta and other fetal tissues.
Gonadotropin is used as an index to ascertain pregnancy and its correct development in the first weeks.
The placenta begins to form already in the very early stages of embryogenesis, to take on a definitive structure around the third month and continue to grow until the end of pregnancy. Its marked endocrine function is mainly aimed at the synthesis of chorionic gonadotropin, estrogen and progesterone.
Similar to what was seen for progesterone, estrogens are secreted both by the corpus luteum, in the earliest stages of pregnancy, and by the placenta in later stages. Unlike those of ovarian origin (where estradiol prevails) the placental estrogens are captained by the estriol, which exhibits a clearly inferior estrogenic activity (compensated, to tell the truth, by the conspicuous secretion).
Main functions of estrogen hormones
- Promote enlargement of the uterus and breasts.
- They stimulate the development of mammary glands and promote the secretion of pituitary prolactin.
Promote the enlargement of the woman's external genitals.
- High levels of estrogen and progesterone suppress the development of other ovarian follicles.
- In synergy with the placental relaxin, they induce the relaxation of the pelvic ligaments in order to make the sacroiliac joints and pubic symphysis more elastic in view of the birth. They also have a clear tendency to enhance uterine contractility, which up to a few weeks before birth is compensated by the proportional increase in progesterone.
- They actively participate in the development of the fetus.
Oxytocin, a hormone secreted by the neurohypophysis that causes the contraction of the uterus at the time of birth, is also worth a mention. During labor, oxytocin is subject to one of the few noticeable positive feedback mechanisms in our body, since the stimulation and stretching of the cervix induces a further release of oxytocin.
After the birth, with the expulsion of the placenta, the hormonal concentrations return to basal levels, except for the prolactin values that are kept high; as the name suggests, this hormone is of crucial importance for lactation (inhibited by estrogen and progesterone during pregnancy). After delivery, milk secretion is stimulated by suction, which promotes both the release of prolactin and oxytocin (this hormone stimulates milk ejection). During pregnancy another hormone - called human chorionic somatomammotropin or placental lactogen hormone - contributes to the preparation of the mammary gland for the subsequent milk secretion, effectively mimicking the biological action of prolactin.
