Swiss from birth
This also applies to children of unmarried Swiss women. The principle further applies to children with a Swiss father who has not been married to their mother, if the father has acknowledged paternity.
However, the birth of the child must be reported to the responsible Swiss representation. This is important as a child born abroad to a Swiss parent who is a citizen of another country loses Swiss citizenship on reaching the age of 25, unless a Swiss authority abroad or in Switzerland has been notified of his or her birth, or he or she has declared in writing that he or she wishes to remain a Swiss citizen.
Here you can find a list of Swiss representations abroad.