Use @born-sex me to define your character's sex as it was designated when they were born. For many characters, this may be the same as the character's present day gender. You don't have to set this if it doesn't apply to you, but you are welcome to do so.
This will be expressed in blood work, though its not going to show up in old blood samples. If you'd like specific karyotypes as part of your character's back story to show up in your blood as part of your biological sex, we can support this via a custom BIOLOGICAL-SEX-CHROMASOMES rp info flag staff can set if you file a @service-request.
I believe NB-at-birth is a viable option!