You just need to give them a months notice before your mat leave. I waited until I was about 6.5 months preggo until I told my boss. I waited because I was still under probation and was worried they might let me go (although it's illegal to let someone go because they're preggo). Tell them when you're comfortable, but sounds like you might need to tell them soon
Hey, I was in a similar boat where I was expecting a permanent position and didn't want to tell them too soon incase they come up with some other excuse to not give it to me. I told my employer. I told my manager around 6 months
Per the provincial/federal guidelines, I think the expectation is that you give your employer 4 weeks notice of when you intend to start your leave. You are not obligated to tell them when you are coming back until 4 weeks prior to that as well. Most employees prefer 6-8 weeks notice. I personally waited until I was at least 20 weeks just for risk mitigation and not wanting to deal with the excitement and then devastation of a loss with my colleagues. But because it took a while for my husband and I to decide when/how we wanted to use our leaves, we ended up only giving our employers 4 weeks notice of when *my* leave would start. For my husband, he told his employee that his leave starts when I deliver, so the start date is not certain
You don't need to tell them until applying for mat leave unless there is a risk to your baby. For me I had to tell them early because we had a covid outbreak