gender identity, while intersex women may have sex characteristics that do not fit typical notions of female biology. Biology Genetic characteristics Typically, the cells of female humans contain two X chromosomes, while the cells of male humans have an X and a Y chromosome. During early fetal development, all embryos have phenotypically female genitalia up until week 6 or 7, when a male embryo's gonads differentiate into testes due to the action of the SRY gene on the Y chromosome. Sex differentiation proceeds in female humans in a way that is independent of gonadal hormones. Because humans inherit mitochondrial DNA only from the mother's ovum, genealogical researchers can trace maternal lineage far back in time. Hormonal characteristics, menstruation and menopause Female puberty triggers bodily changes that enable sexual reproduction via fertilization. In response to chemical signals from the pituitary gland, the ovaries secrete hormones that stimulate maturation of the body, including increased height and weight, body hair growth, breast development and menarche (the onset of menstruation). Most girls go through menarche between ages 12–13, and are then capable of becoming pregnant and bearing children. Pregnancy generally requires internal fertilization of the eggs with sperm, via either sexual intercourse or artificial insemination, though in vitro fertilization allows fertilization to occur outside the human body. Humans are similar to other large mammals in that they usually give birth to a single offspring per pregnancy, but are unusual in being altricial compared to most other large mammals, meaning young are undeveloped at time of birth and require the aid of their parents or guardians to fully mature. Sometimes humans have multiple births, most commonly twins. Usually between ages 49–52, a woman reaches menopause, the time when menstrual periods stop permanently, and they are no longer able to bear children. Unlike most other mammals, the human lifespan usually extends many years after menopause. Many women become grandmothers and contribute to the care of grandchildren and other family members. Many biologists believe that the extended human lifespan is evolutionarily driven by kin selection, though other theories have also been proposed. Morphological and physiological characteristics In terms of biology, the female sex organs are involved in the reproductive system, whereas the secondary sex characteristics are involved in breastfeeding children and attracting a mate. Humans are placental mammals, which means the mother carries the fetus in the uterus and the placenta facilitates the exchange of nutrients and