The woman's menstrual cycle

First half of the menstrual cycle (days 1–14)

With the increased production of FSH, egg maturation begins in the ovaries (ovaries). Each of these eggs lies in an egg sac (follicle), which is responsible for the formation of the female sex hormone (estrogen). This hormone builds up the uterine lining (endometrium) and opens the cervix (cervix) slightly. In addition, it promotes the production of cervical secretions, which makes it easier for the sperm to penetrate the uterine cavity. At some point, the body chooses the largest follicle to grow further and stops the growth of the others. The largest follicle is called the ‘dominant’ follicle. In this way, it is ensured that only one egg ovulates. In some women, however, it is seen that two eggs can mature in the natural cycle. This happens more frequently the older the woman is. Around 1000 eggs fight to become the dominant follicle every month.

Mid-menstrual cycle – Ovulation (approx. 14-16 days)

In the middle of the menstrual cycle, the egg in the follicle is fully matured and produces maximum amounts of estrogen. Due to the increased amount of estrogen, the pituitary gland secretes greater amounts of LH. This sex hormone ensures that the follicle bursts so that the egg comes out of the ovary – the so-called ovulation (ovulation). The corpus luteum is formed from the remains of the follicle, which produces the hormone progesterone. In the event of fertilization, progesterone helps maintain the pregnancy. The mature eggs are picked up by the fallopian tube and transported towards the uterus. The eggs are viable and can be fertilized for approx. 24 hours.

Second half of the menstrual cycle (approximately days 17–28)

The egg can be fertilized while it is in the fallopian tube. If a sperm cell reaches the egg and succeeds in breaking down the egg shell, the egg divides several times even before it reaches the uterus. After another four to five days, the fertilized egg attaches itself to the lining of the uterus. This immediately begins to emit the signal substance hCG (human chorionic gonadotropin, so-called pregnancy hormone) to maintain the pregnancy.

If the egg in the fallopian tube is not fertilized, this is detected by the ovary, as the hCG signals of the uterus are absent. The corpus luteum, which is formed from the follicular remains, perishes after 10-14 days. This means that the progesterone level in the woman’s blood drops and the uterine lining is rejected. Thus begins the first day of the new menstrual cycle.
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