UK law treats sex and gender as the same thing and in general conversation people often use these terms interchangeably. However, when it comes to monitoring questions, it is better to use the term gender rather than the term sex

This will help make it clear to people that you are asking them to tell you about their self-perception of their gender rather than about their biological sex

Asking questions specifically about biological sex at birth will be ineffective - most trans people find this question offensive and will provide their gender identity, making the data collected inaccurate. However,  gender data can still be used to help plan services relating to biological sex, as most people will have a biological sex that corresponds with their gender

The technical term for someone’s self-perception of their gender is their gender identity - it’s the term we use most often in this guide. But it’s fine to use the term gender instead as this is more widely understood

It is absolutely vital to ensure that this isn’t restricted to male and female, but is inclusive of nonbinary identities. There are lots of different non-binary identities -  including a space for individuals to describe their own gender identity is important, and will allow you to capture much more information

Remember that most trans people will identify as men or women - this third option is usually used by people who identify as non-binary (or in other non-binary terms, such as gender-fluid or genderqueer)