Eggs are formed in the gonads (ovaries) of the female fetus during prenatal development. Each girl at birth has about 2 million immature eggs enclosed in follicles.
By the time of menarche (first menstruation), only 400 thousand immature eggs are preserved, the rest die. But that's completely normal. The remaining amount is more than enough for reproductive function.
Menstruation begins in girls after puberty. Some time ago, the age range for menarche was considered to be 13-16 years. Now there is a tendency towards acceleration (accelerated development), so the first menstruation can occur as early as 10-12 years.
The first day of the menstrual cycle is usually called the first day of bleeding. This is a period of active rejection (desquamation) of the old outer layer of the endometrium - the lining of the uterine cavity. Menstrual bleeding normally lasts 3-6 days (each woman has her own way). After the cessation of bleeding under the influence of certain hormones, a new layer of endometrium gradually grows in the uterus.
At this moment, several follicles grow in the ovaries at once.
After the seventh day of the cycle, one follicle (in rare cases more) begins to grow more intensively than the others. Such a follicle is called dominant. The egg inside it matures completely and, around the 14th day of the cycle, leaves the ruptured follicle into the fallopian tube. There she will wait for the sperm (male reproductive cell).
The process of rupture of the dominant follicle and release of the egg is called ovulation.
This will be a mature egg, ready for fertilization. Its viability will remain for 24 hours, after which, if fertilization does not occur, it will die. In the next menstrual cycle the process will repeat again.
After ovulation, what remains of the follicle will turn into a temporary gland - the corpus luteum, which will produce progesterone (the hormone necessary directly for conception and subsequent implantation of the fertilized egg into the wall of the uterus). Meanwhile, in the uterus, active transformations of the grown endometrial layer are taking place, preparing it to receive the embryo.
If fertilization and implantation do not occur, then the level of hormonal activity gradually decreases, and the endometrium that has grown in this cycle is rejected again. A new menstruation is coming. The cycle begins again.