Anne Fausto-Sterling: By birth, then, a baby has five layers of sex. But as with chromosomal sex, each subsequent layer does not always become strictly binary.
[…] Adding to the complexity, the layering does not stop at birth. The adults surrounding the newborn identify sex based on how they perceive genital sex (at birth or from an ultrasound image) and this begins the process of gender socialization. Fetal hormones also affect brain development, producing yet another layer called brain sex. More here.