Based on physical and medical factors, a person is assigned a sex at birth. Most people are assigned male or female, but sex can be more complicated than that. Also, remember that sex and gender are two different things. Planned Parenthood has a great description of what sex is:
“Assigned sex is a label that you’re given at birth based on medical factors, including your hormones, chromosomes, and genitals. Most people are assigned male or female, and this is what’s put on their birth certificates.
When someone’s sexual and reproductive anatomy doesn’t seem to fit the typical definitions of female or male, they may be described as intersex.
Some people call the sex we’re assigned at birth “biological sex.” But this term doesn’t fully capture the complex biological, anatomical, and chromosomal variations that can occur. Having only two options (biological male or biological female) might not describe what’s going on inside a person’s body.
Instead of saying “biological sex,” some people use the phrase “assigned male at birth” or “assigned female at birth.” This acknowledges that someone (often a doctor) is making a decision for someone else. The assignment of a biological sex may or may not align with what’s going on with a person’s body, how they feel, or how they identify.”