It's always possible to be refused entry into Canada on a TRV. So the answer to your question is yes - it's possible. However hopefully very unlikely.
The officers could potentially refused your wife entry if they believe she has plans to stay in Canada and give birth to her baby here. I assume you will be traveling together and will have return tickets. This should show the officers she has no such plans. The fact she has come to Canada on a TRV previously will work in her favour. Just in case you run into any questions, I would bring additional proof that shows you have ties to your country outside of Canada (e.g. property, jobs) and will be returning there after your visit.
I think you should have no issues entering Canada.
Anyway, as Scylla suggested carry all possible evidence that shows she will be back to deliver the baby. For example some hospitals give "confirmation" of caesarean booking. Such things that show you are indeed planning to go back are of great importance.
Thank you both kindly for these reassuring comments and suggestions! We'll definitely bring proof of ownership of our home here as well as international health care coverage, work contracts and birth plans at our local hospital.
