If they're having IVF treatment
Having IVF treatment can be difficult emotionally and physically. It's a good idea to be understanding and supportive towards someone who's having this treatment. For example, they'll probably need time off for medical appointments.
A supportive attitude also means they're more likely to be open with you about how their treatment, and any problems they're having, could affect their wellbeing or work.
An employee having IVF treatment might tell you they could become pregnant, once they've reached the 'embryo transfer' stage. This is the point when a fertilised egg (embryo) is placed inside their body. They do not have to tell you at this stage.
If they've reached the embryo transfer stage
If the employee tells you they've reached the embryo transfer stage, from a legal perspective you must treat them as pregnant.
From this stage they'll have the same rights as any pregnant person (including leave, pay, and protection from discrimination because of pregnancy or maternity).
It can take several more weeks for a pregnancy test to confirm if the pregnancy is successful. They do not have to tell you about the pregnancy's success. But an open and supportive working environment means it's more likely they'll tell you about any issues that could affect their wellbeing or work.