A woman's menstrual cycle determines her fertility and is of great importance for her health and well-being. It's a complicated process: The pituitary gland produces the hormones FSH (follicle stimulating hormone) and LH (luteinising hormone). Under the influence of FSH, an oocyte matures in the woman's ovary. This is located in an oocyte follicle. In the middle of the cycle, the oocyte is released from the follicle by the secretion of LH and released into the fallopian tube. This process is called ovulation. The maturing follicle produces oestrogens, which are responsible for building up the lining of the uterus. After ovulation, the oocyte is capable of fertilisation for about six hours.
Fertilisation of the oocyte takes place in the fallopian tube. A sperm penetrates the oocyte membrane, the paternal chromosomes are released into the oocyte and cell division begins. The migration of the embryo through the fallopian tube into the uterus takes four to five days. Meanwhile, the embryo continues to develop and reaches the uterus as a so-called blastocyst. That's where it eventually nests. By releasing signalling substances to the maternal organism, the function of the corpus luteum, which has developed in the ovary after ovulation, is maintained. The corpus luteum produces the corpus luteum hormone (progesterone), which maintains the pregnancy. If pregnancy does not occur, the corpus luteum stops producing hormones after 14 days. There is menstruation.
The hormones of the pituitary gland act on the female sex hormones and cause an oocyte follicle to grow. The larger the follicle becomes, the closer ovulation is. At the same time, more oestrogens (E2) are produced in the follicle. These hormones build up the lining of the uterus and open the cervix by a few millimetres. They also provide the necessary production of cervical secretions that allow sperm to enter the uterus.
The maximum oestrogen production of the mature follicle leads to the release of LH from the pituitary gland. This sets in motion the final maturation processes of the oocyte that lead to implantation. The fallopian tube on the corresponding side catches the mature oocyte that is ready for fertilisation and transports it towards the uterus.
During this time, the embryo nests in the appropriately prepared endometrium. The embryo sends out hormone signals to tell the ovary to implant. The corpus luteum remains intact and produces the hormone progesterone. This is responsible for the maintenance of pregnancy.
The oocyte in the fallopian tube has not been fertilised. The ovary detects this because there are no hCG hormone signals coming from the uterus. A corpus luteum is formed, but it disappears after about ten to 14 days. This causes the level of the corpus luteum hormone (progesterone) in the blood to drop and the lining of the uterus to bleed. The 1st day of the new cycle has begun.