Not sure how much experience you have with immigration but we routinely see people denied when they mention they are pregnant because applications often say they are planning on helping with cooking, cleaning, caring for the baby, etc. which can be considered illegal work. Grandparents are likely to want to stay longterm and a baby creates stronger ties to Canada. A supervisa is intended for more of a long visit. It is hot or miss some get approved some get denied. In Canada fathers are much more active in caring for children and doing domestic chores and are expected to support their wives in pregnancy, delivery and with the care of the baby afterwards.My take on this is it's best to be honest and forthcoming with information like your wife's pregnancy, rather than hide and have them discover this easily-available information in another way. In my experience, the visitor visa category can be very arbitrary, and the reasons why they may accept or reject can be difficult to divine.
Props to you for your honesty, good luck with the application!