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In terms of biology, the female sex organs are involved in the reproductive system, whereas the secondary sex characteristics are involved in nurturing children or, in some cultures, attracting a mate. The ovaries, in addition to their regulatory function producing hormones, produce female gametes called eggs which, when fertilized by male gametes (sperm), form new genetic individuals. The uterus is an organ with tissue to protect and nurture the developing fetus and muscle to expel it when giving birth. The vagina is used in copulation and birthing (although the word vagina is often colloquially and incorrectly used for the vulva or external female genitalia, which also includes the labia, the clitoris, and the female urethra). The breast evolved from the sweat gland to produce milk, a nutritious secretion that is the most distinctive characteristic of mammals, along with live birth. In mature women, the breast is generally more prominent than in most other mammals; this prominence, not necessary for milk production, is probably at least partially the result of sexual selection. (For other ways in which men commonly differ physically from women, see man.) During early fetal development, embryos of both sexes appear gender-neutral. As in cases without two sexes, such as species that reproduce asexually, the gender-neutral appearance is closer to female than to male. A fetus usually develops into a male if it is exposed to a significant amount of testosterone (typically because the fetus has a Y chromosome from the father). Otherwise, the fetus usually develops into a female, typically when the fetus has an X chromosome from the father, but also when the father contributed neither an X nor Y chromosome. Later at puberty, estrogen feminizes a young woman, giving her adult sexual characteristics. An imbalance of maternal hormonal levels and some chemicals (or drugs) may alter the secondary sexual characteristics of fetuses. Most women have the karyotype 46,XX, but around one in a thousand will be 47,XXX, and one in 2500 will be 45,X. This contrasts with the typical male karotype of 46,XY; thus, the X and Y chromosomes are known as female and male, respectively. Because humans inherit mitochondrial DNA only from the mother's ovum, genetic studies of the female line tend to focus on mitochondrial DNA. Whether or not a child is a female does not always determine whether or not the child later will identify themselves that way (see gender identity). For instance, intersex individuals, who have mixed physical and/or genetic features, may use other criteria in making a clear determination. At birth, babies may be assigned a gender based on their genitalia. In some cases, even if a child had XX chromosomes, if they were born with a penis, they were raised as a male.[22] There are also transgender or transsexual women, who were born or physically assigned as male at birth, but identify as women; there are varying social, legal and individual definitions with regard to these issues (see trans woman). "The Life & Age of Woman - Stages of Woman's Life from the Cradle to the Grave",1849 Although fewer females than males are born (the ratio is around 1:1.05), due to a longer life expectancy there are only 81 men aged 60 or over for every 100 women of the same age. Women typically have a longer life expectancy than men.[23] This is due to a combination of factors: genetics (redundant and varied genes present on sex chromosomes in women); sociology (such as not being expected in most countries to perform military service); health-impacting choices (such as suicide or the use of cigarettes, and alcohol); the presence of the female hormone estrogen, which has a cardioprotective effect in premenopausal women; and the effect of high levels of androgens in men. Out of the total human population, there are 101.3 men for every 100 women (source: 2001 World Almanac). Girls' bodies undergo gradual changes during puberty. Puberty is the process of physical changes by which a child's body matures into an adult body capable of sexual reproduction to enable fertilisation. It is initiated by hormonal signals from the brain to the gonads. In response to the signals, the gonads produce hormones that stimulate libido and the growth, function, and transformation of the brain, bones, muscle, blood, skin, hair, breasts, and sexual organs. Physical growth—height and weight—accelerates in the first half of puberty and is completed when the child has developed an adult body. Until the maturation of their reproductive capabilities, the pre-pubertal, physical differences between boys and girls are the genitalia, the penis and the vagina. Puberty is a process that usually takes place between the ages 10–16, but these ages differ from girl to girl. The major landmark of girls' puberty is menarche, the onset of menstruation, which occurs on average between ages 12–13.[24][25][26][27] Woman nursing her infant Most girls go through menarche and are then able to become pregnant and bear children.[28] This generally requires internal fertilization of her eggs with the sperm of a man through sexual intercourse, though artificial insemination or the surgical implantation of an existing embryo is also possible (see reproductive technology). The study of female reproduction and reproductive organs is called gynaecology. |
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