none of what others are telling you are true! before you believe what others say, ask yourself, "how would they know?" "what experience do they have with sponsoring a spouse?" "do they work for CIC and process applications?"
If you are expecting to give birth during the application process, then you will need to notify CIC of the change to your family. I am not sure how it works if the child is born in canada, and is technically a canadian citizen. CIC definitely needs to know about the baby, so that you guys are not found to be misrepresenting yourselves. You can even write a letter in your application letting them know you are pregnant, when you are due, and plan to update your application as soon as the baby is born. Many people go through this, so hopefully someone with more experience can give you more details about it.
Also, if you are getting married, i am wondering why you are bothering with the common-law application. From what I've read, applying for spousal sponsorship is easier and requires less leg work for the application.