Considering sharing pregnancy news at work

I'm considering telling my boss this week. I'll be 11 weeks on Friday and we have our first ultrasound tomorrow morning to confirm everything is okay.

Career wise - I'm in a tough spot right now. I'm 7 months into a 1 year contract that will end in December, the same time I'm due. I'm prepared that I likely will have no permanent role to return to at my company once I share the news I'm pregnant as it could be difficult to be hired permanently before the baby arrives. I'm in Canada, so I will still qualify for a paid leave for 1 year and will have the opportunity to job search during that time. Not ideal to have to job search, but I feel fortunate to have access to some support. 

I'm most worried about no longer being taken seriously at work, or my boss writing me off once he knows I have a maternity leave looming in my future.

Any advice? Have you had a positive reaction at work?