This is a tough one.
Ideally – it’s no one’s business – so when you’re ready.
Your legal position
Legally (in Australia, specifically), you must tell your employer that you plan on taking parental leave at least 10 weeks before your last work day. So if you plan on taking leave from 37 weeks – you have to tell them about your plans by the time you’re 27 weeks pregnant.
If you’re employed in Australia, the law protects you against discrimination. According to the Sex Discrimination Act 1984, the Fair Work Act 2009 and other state and territory specific legislation – you can’t be treated unfairly because you’re pregnant – eg. you can’t be fired, made to work less hours or given less important work or overlooked for promotion or training.
Accessing parental leave and flexible working arrangements depends on how long you’ve been at your job and your workplace policies.
When you tell your employer is really up to you.
If you have morning sickness and it is affecting your punctuality or people are asking questions – it may be best to let a few people in on the secret just so they can cover for you if need be. For me, I told my secretary, my supervising partner and a few of my most trusted colleagues after I had my dating scan at around the 7 week mark. They sort of clued in to the fact that things weren’t right with me because I was showing up to work a bit later than usual and feeling very ill for most of the day (morning sickness is a misleading descriptor and should really be renamed “all day sickness”).
There will be a few antenatal appointments that you have to attend which can often cut into your work day. When this started to happen for me, I let HR know.
If your office has flexible working arrangements – you’ll probably have the ability to can keep it under wraps for as long as you feel comfortable.