The sexual development of the girl is a complex process of sexual maturation, accompanied by a number of physiological, structural, functional changes of the female genital organs, development of secondary sexual characteristics and psychological changes leading to the search and awareness of the self. The changes in the girl’s body are the result of genetic, endocrine, hygiene-dietary, etc. factors. From birth to full sexual maturity, sexual development goes through several main stages. Embryonic development of the female reproductive system begins in the 7th week of gestation, during which male and female differentiation takes place. The sex-determining factor (SRY) is of primary importance for the differentiation of the sexes, the presence of which determines male sex, and its absence – female. The female reproductive system consists of external and internal sex organs. The internal genital organs include the uterus, fallopian tubes, ovaries, vagina, and the external ones include the labia minora and labia majora, clitoris, pubic eminence, vaginal vestibule, and large vestibular glands. The ovary is one of the most important organs of the female reproductive system, as it produces eggs and secretes the female sex hormones leading to sexual maturation. In the ovaries of a newborn girl, there are about 400,000 primary follicles, and in the process of sexual maturation, a very small part of them manage to mature and become mature eggs – about 400-500. Conditionally, the time from the birth of the girl to her sexual maturity can be divided into three periods: 1. Neonatal period – covers the period from birth to 2-3 weeks after it. During this period, a hormonal crisis is observed, expressed with increased estrogen stimulation and the influence of the remaining placental and maternal hormones. During this period, the newborn’s labia are soft and red, the vagina is upright, the uterus is high between the promontorium and the symphysis, and the cervix is ​​twice the size of the uterine body. Acidophilic bacteria are found in the vagina – the pH is about 5. With a decrease in the levels of maternal hormones in the newborn at 2-10%, the so-called Halban reaction – bleeding caused by endometrial atrophy. Usually this bleeding lasts for 3-4 days. In the ovaries of the newborn, follicles of varying degrees of maturity are found, but not mature ones. Under the influence of maternal and placental hormones, colostrum-like fluid may leak from the baby girl’s breast. 2. Latent period – covers the period from the exhaustion of maternal hormones to the onset of puberty. During the latent period, the secretion of ovarian hormones is minimal, which is why the appearance of secondary birthmarks is not observed. No lactic acid bacteria are found in the vagina, which is why the environment is slightly alkaline – the protective function of the vagina is impaired. By the second year of the girl, the uterus decreases its size (involutes), but by the age of eight it returns to the size it had at birth.Continuous maturation and atresia of primordial follicles is observed in the ovaries. 3. Puberty period – a period of actual sexual maturation. It, in turn, is divided into three sub-periods: Pre-puberty – from 9 to 11 years of age. It is characterized by secretion of hormones from the pituitary gland and increased secretion of female sex hormones – at first estradiol, and later estradiol and estrone. Under the influence of estrogens, fatty tissue accumulates in the pubic region, the labia grow and cover the vestibule of the vagina, the vagina elongates and the typical acidophilic flora develops in it, providing a protective acidic environment. The uterus increases in size, and the ratio between the cervix and the uterine body becomes 1:1. Development of secondary sexual characteristics occurs – thelarche (development of breasts), pubarche (development of pubic hair) and axillary (development of hair in the axillary pit). The Tanner classification is used to assess their development. Puberty period – period of appearance of first menstruation – menarche. On average, girls first menstruate between 10-12 to 16 years of age, depending on physiological, racial, ethnic, and environmental factors. It has been established that the first menstruation occurs when the secondary birthmarks of Tanner grade 3-4 develop. Usually, the first menstruation is pseudomenstruation – without the release of a mature egg, because the endometrium does not transform into a secretory phase. Post-puberty period – the period from the first menstruation (menarche) to the end of puberty, which occurs until about 16-18 years of age. The external and internal genitalia are morphologically and functionally formed finally around the age of 16. At this age, the girl is physiologically prepared for possible fertilization and pregnancy, although the mental and social preparation at this age is not sufficiently developed to perform reproductive functions. Puberty is a long and complex process of morphological, physiological and emotional changes in the girl’s body, which not only physically prepares her but also mentally to realize her sexuality and the opportunity to realize her reproductive potential.Development of secondary sexual characteristics occurs – thelarche (development of breasts), pubarche (development of pubic hair) and axillary (development of hair in the axillary pit). The Tanner classification is used to assess their development. Puberty period – period of appearance of first menstruation – menarche. On average, girls first menstruate between 10-12 and 16 years of age, depending on physiological, racial, ethnic, and environmental factors. It has been established that the first menstruation occurs when the secondary birthmarks of Tanner grade 3-4 develop. Usually, the first menstruation is pseudomenstruation – without the release of a mature egg, because the endometrium does not transform into a secretory phase. Post-puberty period – the period from the first menstruation (menarche) to the end of puberty, which occurs until about 16-18 years of age. The external and internal genitalia are morphologically and functionally formed finally around the age of 16. At this age, the girl is physiologically prepared for possible fertilization and pregnancy, although the mental and social preparation at this age is not sufficiently developed to perform reproductive functions. Puberty is a long and complex process of morphological, physiological and emotional changes in the girl’s body, which not only physically prepares her but also mentally to realize her sexuality and the opportunity to realize her reproductive potential.Development of secondary sexual characteristics occurs – thelarche (development of breasts), pubarche (development of pubic hair) and axillary (development of hair in the axillary pit). The Tanner classification is used to assess their development. Puberty period – period of appearance of first menstruation – menarche. On average, girls first menstruate between 10-12 to 16 years of age, depending on physiological, racial, ethnic, and environmental factors. It has been established that the first menstruation occurs when the secondary birthmarks of Tanner grade 3-4 develop. Usually, the first menstruation is pseudomenstruation – without the release of a mature egg, because the endometrium does not transform into a secretory phase. Post-puberty period – the period from the first menstruation (menarche) to the end of puberty, which occurs until about 16-18 years of age. The external and internal genitalia are morphologically and functionally formed finally around the age of 16. At this age, the girl is physiologically prepared for possible fertilization and pregnancy, although the mental and social preparation at this age is not sufficiently developed to perform reproductive functions. Puberty is a long and complex process of morphological, physiological and emotional changes in the girl’s body, which not only physically prepares her but also mentally to realize her sexuality and the opportunity to realize her reproductive potential.