The progression from fetal development to aging in females represents a gradual physiological process. It also reflects the occurrence, development, maturation, and eventual decline of the hypothalamic-pituitary-ovarian axis. A woman’s life can be divided into seven stages based on age and physiological characteristics, although these stages lack absolute boundaries and may vary due to factors such as genetics, environment, and nutrition.
Fetal Period
The zygote is a new individual formed by 23 pairs (46 individual) chromosomes from paternal and maternal origins, including one pair of sex chromosomes, which determine sexual development. A zygote with XX chromosomes develops into a female, while one with XY chromosomes develops into a male. The differentiation of primitive gonads occurs more slowly in females; by the 8th–10th week of embryonic development, the gonadal tissue begins to exhibit ovarian structure. The primordial germ cells differentiate into oogonia, which further develop into primary oocytes. The cortical cells of the sex cords surround the primary oocytes to form primordial follicles, also known as primary follicles. Following the formation of the ovaries, the absence of androgens and Müllerian inhibiting factor (MIF) leads to the regression of the mesonephric ducts (Wolffian ducts) and the development of the paramesonephric ducts (Müllerian ducts) into the fallopian tubes, the uterus, and the upper vagina. The urogenital sinus forms the lower segment of the vagina. In the absence of androgens, the external genitalia naturally differentiate into female anatomy.
Neonatal Period
The neonatal period refers to the first four weeks after birth. During fetal life, female fetuses are influenced by maternal and placental estrogens, resulting in relatively full-looking external genitalia and slightly enlarged or lactating breasts in newborns. After birth, as the baby separates from the maternal environment, circulating estrogen levels decline rapidly, which may lead to minor vaginal bleeding. These physiological changes typically resolve naturally over a short period.
Childhood
The period from four weeks to 12 years of age is considered childhood. In early childhood (before 8 years), the function of the hypothalamic-pituitary-ovarian axis is suppressed due to the hypothalamus and pituitary’s heightened sensitivity to the negative feedback effects of low estrogen levels (≤10 pg/ml) and central inhibitory signals. During this stage, the reproductive system remains immature. The vagina is narrow with thin epithelium, lacks rugae, and contains cells that are glycogen-poor, leading to low acidity and weak anti-infective defenses, making it more prone to inflammation. The uterus is small, with the cervix making up approximately two-thirds of its total length, and the myometrium is thin. The fallopian tubes are long and thin, while the ovaries are elongated and narrow. Although follicles can undergo autonomous growth (independent of gonadotropins), they only develop up to the pre-antral stage before regressing and degenerating. At this stage, the uterus, fallopian tubes, and ovaries are located in the abdominal cavity.
In later childhood (approximately after age 8), suppression of hypothalamic gonadotropin-releasing hormone (GnRH) begins to wane. Ovarian follicles start developing under the influence of gonadotropins from the pituitary and secrete sex hormones to some extent, though not yet at fully mature levels. The ovaries become oval in shape, and the uterus, fallopian tubes, and ovaries gradually descend into the pelvic cavity. Subcutaneous fat begins to accumulate in the chest, hips, shoulders, and pubis, and breast development commences, giving rise to initial observable female characteristics.
Adolescence (Puberty)
Adolescence represents the transition from childhood to adulthood, during which the reproductive organs, endocrine system, and physical characteristics gradually develop to maturity. According to the World Health Organization (WHO), adolescence spans from 10 to 19 years of age. The onset of puberty typically occurs between 8 and 10 years when central inhibition of GnRH is lifted, allowing it to be secreted in a pulsatile manner. This triggers increased levels of gonadotropins and ovarian sex hormones, the emergence of secondary sexual characteristics, and ultimately the attainment of functional reproductive maturity. The timing of puberty is primarily determined by genetic factors and may also be influenced by geography, physical constitution, nutritional status, and psychological factors.
The first sexual characteristic to develop in women is ovarian enlargement under the influence of gonadotropins. Ovarian follicles begin to develop and secrete estrogen, which promotes the transformation of the reproductive system from an immature form to an adult form. The mons pubis thickens, and the labia majora and labia minora become more prominent with pigmentation. The length and width of the vagina increase, the vaginal mucosa thickens and develops rugae, and the uterus enlarges, with a notable increase in the uterine body. The uterine body-to-cervix ratio becomes 2:1. The fallopian tubes thicken and become less convoluted, with the appearance of folds and cilia in their mucosal lining. The ovaries increase in size, containing follicles at various stages of development, giving them a slightly nodular surface. Although some level of fertility emerges at this stage, the reproductive system’s functionality remains incomplete.
Secondary sexual characteristics include a higher-pitched voice, breast development, the appearance of pubic and axillary hair, a greater transverse diameter of the pelvis compared to the anteroposterior diameter, and increased subcutaneous fat in the chest, shoulders, and pelvis, all of which contribute to the female physique.
Adolescence typically spans four distinct overlapping stages, taking approximately 4.5 years to complete:
Thelarche (Breast Development)
This is the initial indicator of secondary sexual development. Most girls experience breast development around the age of 10, achieving full maturity in approximately 3.5 years.
Adrenarche (Onset of Adrenal Function)
Maturation of the adrenal cortex leads to increased secretion of adrenal androgens, resulting in the initial growth of pubic hair, followed by axillary hair about two years later. During this stage, serum levels of dehydroepiandrosterone (DHEA), dehydroepiandrosterone sulfate (DHEAS), and androstenedione rise, while the activities of adrenal 17α-hydroxylase and 17,20-lyase increase, signifying a functional hypothalamic-pituitary-adrenal androgen axis.
Growth Spurt
Around the ages of 11–12, physical growth accelerates, with an average height increase of 9 cm per year. After menarche, growth slows. The growth spurt during puberty results from increased secretion of estrogen, growth hormone (GH), and insulin-like growth factor-1 (IGF-1).
Menarche
The first menstrual period marks a significant milestone in puberty and typically occurs about 2.5 years after the onset of breast development. Menarche indicates sufficient estrogen production by the ovaries to induce endometrial proliferation. Menstrual bleeding occurs once estrogen levels reach a certain threshold and exhibit significant fluctuations. However, the positive feedback mechanism of the central system toward estrogen remains immature at this stage, so even with mature follicular development, ovulation does not always occur. As a result, menstrual cycles are often irregular, and it may take 5–7 years for regular ovulatory cycles to establish.
Adolescence is also associated with significant psychological changes, including the emergence of sexual awareness, heightened emotions, and noticeable changes in intelligence, imagination, and judgment.
Sexual Maturity Period
Also referred to as the reproductive period, this stage is characterized by the peak of ovarian reproductive and endocrine functions. It generally begins around the age of 18 and lasts for approximately 30 years. During this time, female sexual function is at its peak, ovarian function is fully mature, and the ovaries secrete sex hormones, supporting regular cyclic ovulation. Under the influence of ovarian sex hormones, the reproductive organs and breasts undergo cyclical changes. However, starting at around 35–37 years of age, the number of primordial follicles decreases sharply, and the reproductive capability of the ovaries significantly declines with age, typically beginning to show signs of regression around the age of 40.
Menopausal Transition Period
This stage refers to the period from the onset of menopausal symptoms to the final menstrual cycle. It may begin as early as age 40 and can last between 1–2 years or extend up to 10–20 years. During this period, ovarian function gradually declines, with a marked reduction in the number of follicles and an increased likelihood of incomplete follicular development. As a result, menstrual cycles become irregular and often anovulatory. Ultimately, ovarian function ceases due to the natural depletion of ovarian follicles or the remaining follicles losing responsiveness to pituitary gonadotropins. This leads to ovarian failure and the permanent cessation of menstruation, known as menopause. The median age for menopause in women is 49 years, with 90% experiencing menopause between 45 and 55 years of age, and most transitioning between 48 and 52 years of age. While life expectancy has increased significantly, the age of menopause has remained relatively stable, suggesting that menopause age is primarily determined by genetic factors.
In 1994, the World Health Organization (WHO) recommended discontinuing the term "climacteric" and introduced "perimenopausal period" to describe the time from ovarian function decline to within one year after menopause. During the perimenopausal period, the reduction in estrogen levels can lead to vasomotor disturbances and neuropsychological symptoms, manifesting as hot flashes, sweating, mood instability, irritability, depression, restlessness, and insomnia, collectively referred to as menopausal syndrome.
Postmenopausal Period
This period refers to the stage of life following menopause. In the early postmenopausal phase, although the ovaries cease estrogen production, the ovarian stroma continues to secrete small amounts of androgens, which are peripherally converted to estrone, the primary estrogen in the body during this stage. Generally, after the age of 60, the aging process accelerates, transitioning women into the senescence phase. During this time, ovarian function is completely exhausted, and estrogen levels fall to a point insufficient to maintain secondary sexual characteristics. The reproductive organs undergo further atrophy and aging. Abnormal bone metabolism during this stage can lead to osteoporosis, increasing the risk of fractures.