Can true sex be determined at birth?
No, no one can determine “true” sex at birth. Intersex children are born with a body that does not fit into society’s image of male or female. This means that at birth a gender is assumed and at the same time it is assumed that the child will develop a corresponding gender identity. In about 5% of children, that assumption turns out to be incorrect. It is impossible to predict who will be among that five percent. So no, the “true” sex of an intersex child cannot be predicted.
However, it does appear that the assumptions about gender and the gender identity that has yet to be developed are more accurate in some forms of sex diversity than in others. For example, most people with Klinefelter’s Syndrome identify as male, while people with Turner’s Syndrome or Complete Androgen Insensitivity Syndrome tend to identify as female.
Because it is impossible to determine in whom the assumed sex corresponds to the experienced sex, it is very unwise to put children through medical treatments to “confirm” the assumed sex shortly after birth. According to the United Nations, the Council of Europe, the Parliamentary Assembly of the Council of Europe, and the European Parliament, such treatments are a serious violation of the rights of the child and should be prosecuted. In some countries, the government believes that parents should be allowed to decide with doctors whether such treatment should take place.