>>3551283 lb
https://thehill.com/homenews/administration/412418-trump-administration-considering-narrowing-legal-definition-of-gender
A much more solid definition of gender would rely on a chromosome count. God/Nature assigned XY chromosomes to males and XX to females. These are present in every cell nucleus in the human body. Very, very occasionally there will be an extra X or Y chromosome for males or an extra X chromosome for females (see links below). Chromosomal analysis is the only way to definitely tell what's going on because in rare cases external genitalia isn't definitive. All hospital births require a blood test of the baby for various tests, so it shouldn't be too big of a deal to look for an extra chromosome while they're at it.
"The types of newborn screening tests that are done vary from state to state. By April 2011, all states reported screening for at least 26 disorders on an expanded and standardized uniform panel. The most thorough screening panel checks for about 40 disorders. However, because phenylketonuria (PKU) was the first disorder for which a screening test developed, some people still call the newborn screen 'the PKU test'."
https://medlineplus.gov/ency/article/007257.htm
https://ghr.nlm.nih.gov/condition/klinefelter-syndrome
https://ghr.nlm.nih.gov/condition/triple-x-syndrome
https://ghr.nlm.nih.gov/condition/47xyy-syndrome
https://genetic.org/
https://rarediseases.org/rare-diseases/xyy-syndrome/