In the first half of the female menstrual cycle, one to three eggs mature in the ovaries of a healthy woman. In this phase two hormones, the follicle-stimulating hormone (FSH)(FSH) and the luteinising hormone (LH) play a decisive role. Both are formed by the pituitary gland and released into the bloodstream. This in turn is controlled by so-called releasing hormones, which are released by the hypothalamus (diencephalon). The FSH stimulates and supports the egg cell growing in the follicle of the ovaries. The follicle forms female sex hormones (oestrogens) which cause the build-up of the endometrium. Towards the middle of the cycle, the LH content in the blood increases sharply. This hormonal signal causes the bursting of the follicle, the so-called (ovulation), and thus the escape of the egg from the ovary.
The remaining part of the follicle now becomes the (corpus luteum). This forms oestrogens and progesterone, a hormone that prepares the lining of the uterus for the implantation of an embryo. During ovulation, the egg cell enters the fallopian tube, where it is susceptible to the ingestion of a sperm within the following 24 hours. By the penetration of a sperm cell into the egg cell this is fertilised. Paternal and maternal genes merge. The fertilised egg now migrates down the fallopian tube into the uterus. It divides several times and is already called an embryo. After 4 - 5 days, the embryo nests in the lining of the uterus and pregnancy begins.
The pregnancy plant (embryo and placenta) now forms the pregnancy hormone hCG (humanes Choriongonadotropin) human chorionic gonadotropin) and signals the progesterone productionthe corpus luteum via the blood circulation. A pregnancy can therefore be detected by the detection of hCG in the blood.
If the egg cell is not fertilised or if an embryo is not implanted, the corpus luteum stops producing hormones and regresses. The absence of oestrogen-progesterone signals leads to the uterine lining being expelled with the next menstrual period. A new cycle begins.