Research has concluded that the chromosome test historically used to determine sex-at-birth (in which embryos with XX chromosomes are identified as girls and embryos with XY chromosomes are identified as boys) isn't always accurate. Certain regulatory sequences in DNA enhance the development of an embryo's sex organs, making some sex tests more complicated.
A study in the journal Nature Communications concluded that so-called genetic “enhancers” control the development of testes and ovaries. Typically, the SRY gene and the Sox9 gene, both found on the Y chromosome, interact to develop testes. High Sox9 levels are needed for normal testis development. The research identified some cases where XX embryos had additional Sox9 copies and developed testes. Some XY embryos that had fewer Sox9 levels developed ovaries instead of testes.