My Tegan Syndod (her Welsh names translates as "little beautiful surprise") is the daughter of a supposedly safe over 5 weeks post-op boar (not one of mine). It can really happen that late, and she is not the only one I have heard of since.
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All good UK rescues practice a 6 weeks post-op wait, whether that is for their own neutered boars or cross gender bondings. Despite several hundred boars being neutered in the intervening years, I have never heard of any babies - and if that happened, it would make the rounds like a wildfire!
Best of luck!