Does a known sperm donor risk being found a legal parent later without a written agreement?

Yes — under Ontario's parentage rules, a person who provides sperm, eggs, or embryos for another person's reproduction is generally not considered a parent only if there is a written agreement, made before conception, stating that the donor does not intend to be a parent. Without that agreement, a known donor who has an ongoing personal relationship with the recipient parent, or who becomes involved in the child's life, is in a much weaker position to say later that he was "just a donor" — a court asked to determine parentage will look at the parties' actual intentions and conduct, and the absence of a written agreement removes the clearest evidence of what was intended.

This is a common trap for people who arrange donor conception informally between friends or through private arrangements rather than through a fertility clinic, where the paperwork is usually built into the process. Anyone acting as a known donor, or anyone receiving donated sperm from someone they know personally, should have a written agreement signed before conception that clearly states the donor's intended role, and get independent legal advice before signing it.