Upvote:3
Your ‘visa run’ plan will probably look suspiciously like an intention to live in the UK for extended periods through frequent or successive visits, which is against Immigration Rule V4.2(b).
If she is questioned on attempting re-entry after spending 6 months in the UK followed by such a short time out of it, your fiancée may well need to prove a compelling reason to leave the UK (eg a job waiting for her in Canada) in order to convince the Border official that she won’t just stay there with you (and, presumably, her baby in due course).
If she can’t convince them, she will likely be denied entry or given a much shorter stay than the usual 6 months. If that happens, her travel history to the UK as a visa-free national will be damaged and could cause problems on future visits.
Source: https://www.gov.uk/guidance/immigration-rules/immigration-rules-appendix-v-visitor