A strict gender binary is a societal construct, not a biological one. Numerous societies recognize gender beyond a binary. Some recognize as many as seven. Now science is shedding more light on the concept that supports gender as a spectrum instead of two sides of a coin.
In the dominant society labeling as a "boy" or "girl" begins before we're even born. But with all of the factors that contribute to gender, such simplistic labels may some day become obsolete.
Determination of biological sex begins at conception and continues to change through puberty up to adulthood. In a recent Scientific American article, five factors were identified that determine gender: chromosomes, genes, hormones, internal and external sex organs, and secondary sex characteristics.
In an additional layer of complexity, the gender with which a person identifies does not always align with the sex they are assigned at birth, and they may not be wholly male or female."
Intersex individuals, those who bear traits assigned to both ends of the gender spectrum, are the result of a diverse range of conditions, such as 5-alpha reductase deficiency (highlighted in below graphic). The graphic displays just a small cross-section of the pathways of gender determination.
Influences on gender and sex from conception through adulthood beyond gender binary.
(Credit: Pitch Interactive and Amanda Montañez; Source: Research by Amanda Hobbs; Expert review by Amy Wisniewski University of Oklahoma Health Sciences Center)
Given the complexity of each of these factors, variations in any from one individual to another produces gender as a spectrum of characteristics, expressions and identities beyond a simple gender binary.
Several countries recognize non-binary gender designations on legal identification and birth certificates. The state of Oregon became the first state in the U.S., followed by measures in Washington D.C., California, and New York. Perhaps some day in the near future, "to be determined" will replace the traditional gender announcement for children.