Y Chromosome = Man wrote:
My only suggestion: Rather than using the Birth Certificate, I would say:
"People with a Y chromosome shall be excluded from Woman's sports."
Beyond the expense of testing every person on a women's team, the reason that that might not work--and Gabbard may well have taken this into consideration--is that there are biological XY individuals who have complete androgen insensitivity syndrome (CAIS) and would typically be certified as female at birth.
CAIS means they cannot respond to testosterone and so develop as females, with female genitalia, normal breast growth, and so on. The condition is often not found until puberty: CAIS individuals have no uterus and can't menstruate.
Some CAIS women have competed at the Olympic level. They may be taller than the average for women but shorter than average for men. And because they are insensitive to testosterone they don't have the advantage that a typical male-to-female trans athlete could have.
https://en.wikipedia.org/wiki/Complete_androgen_insensitivity_syndrome