Do I have to notify my employer as soon as I find out I’m pregnant?

My roster has me working from 8 am to 5 pm on four weekdays, which includes 30 minutes of admin time and a 30 minute lunch break on these days. I also work one half-day a week as well. I am at work for 40 hours per week, but I am only being paid for 38 hours. Is this correct?

My current employment has become untenable for a multitude of reasons. How do I go about ending my employment agreement?