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Biology

Embryos that inherit two X chromosomes (XX), one from each parent, are generally identified as girls when born.

About one in a thousand girls have a 47,XXX karyotype, and one in 2500 have a 45,X one.

Girls typically have a female reproductive system. Some intersex children with ambiguous genitals may be classified as girls, while some Transgender youth have a gender identity as girls.

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 fertilization. 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. Puberty is a process that usually takes place between 10 and 16 years, but these ages differ from person to person. The major landmark of female puberty is menarche, the onset of menstruation, which occurs on average between 12 and 13. According to a 2010 Canadian study, the variation of age in which menstruation begins had a "statistically significant" relation to where the child was living, household income, and family type.