waste between the mother and fetus. The ovaries, in addition to their regulatory function of producing hormones, produce female gametes called ova 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 term vagina is often colloquially and incorrectly used in the English language for the vulva (or external female genitalia), which consists of (in addition to the vaginal opening) the labia, the clitoris, and the female urethra. The mammary glands are hypothesized to have evolved from apocrine-like glands 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 thought to be at least partially the result of sexual selection. Estrogens, which are primary female sex hormones, have a significant impact on a female's body shape. They are produced in both men and women, but their levels are significantly higher in women, especially in those of reproductive age. Besides other functions, estrogens promote the development of female secondary sexual characteristics, such as breasts and hips. As a result of estrogens, during puberty, girls develop breasts and their hips widen. Working against estrogen, the presence of testosterone in a pubescent female inhibits breast development and promotes muscle and facial hair development. Gender distribution and life expectancy Although girls are born slightly less frequently than boys (the ratio is around 1:1.05), newborn girls are more likely to reach their first birthday than are boys, and women typically have a longer life expectancy by six to eight years, although in some areas discrimination against women has lowered female life expectancy to less than or equal to that of men. Out of the total human population in 2015, there were 1018 men for every 1000 women. The differences in life expectancy are partly due to inherent biological advantages, but also reflect behavioral differences between men and women. The gap is narrowing to some extent in some developed countries, possibly due to increased smoking among women and declining rates of cardiovascular disease among men. The World Health Organization (WHO) writes that it is "important to note that the extra years of life