It is not mandatory to have a private medical insurance plan for anybody in Canada. It would be a good idea for anybody to have a minimal emergency insurance in case they get hit by a car or something but for a pregnant woman to buy expensive insurance that will not cover her pregnancy because it's pre-existing would not make any sense.
You do not have to necessarily go to a walk in clinic to see a doctor. You can ask people you know which doctor they are going to and contact their office to see if they are taking new patients but you will still have to pay. Seeing a regular doctor who has a small practice and doesn't get a lot of people who are without insurance could be less expensive for you than going to a walk-in. The walk-ins are prepared for people who don't have insurance so they already have their prices listed and most of them are high. A regular doctor is more likely to charge modest prices.